By Sarah E. Needleman and Tripp Mickle

At Apple Inc.'s software conference two years ago, Ken Case peppered peers over breakfasts with questions about their businesses. He said the conversations helped his company avoid mistakes that could have cost thousands of customers and millions of dollars.

This year, Mr. Case will be attending Apple's Worldwide Developers Conference, which begins Monday, from his home in Seattle. Because of the coronavirus crisis, Apple is hosting the event entirely online for the first time since it was first held more than three decades ago. Mr. Case said he will miss the meals and serendipitous meetings that usually fill a week in California annually.

"There's a value in meeting strangers, and that doesn't happen virtually, " said Mr. Case, whose company makes productivity software for Apple products.

Apple in the 1980s helped spawn tech conferences that came to define the innovation and marketing profile of Silicon Valley. In recent years these events have grown in scale and glitz, and include Microsoft Corp.'s Build, Salesforce.com Inc.'s Dreamforce and Amazon.com Inc.'s AWS re:Invent. About two dozen of the conferences are held annually, drawing more than a million in-person attendees combined.

Now tech companies are trying to create worthy virtual versions of events for the software developers, business partners and customers powering their operations by broadcasting keynote speeches, question-and-answer sessions and more over live video. Event organizers are trying to keep participants as engaged as possible, a challenging endeavor now that people are spending more time on screens, and take advantage of reaching larger audiences at a lower cost.

Nine weeks before its annual Build conference for developers, Microsoft decided to turn the event into an entirely virtual one. The two-day online gathering last month had a larger-than-expected audience with 193,000 attendees, up from 6,200 who participated in Seattle last year, said Bob Bejan, Microsoft's head of global events.

In moving Build online, Microsoft didn't try to replicate past events, because watching a series of speakers talk into a camera would be "boring as heck," Mr. Bejan said. The company, which this year waived the full roughly $1,500-a-person fee it charged last year, instead created a new format with two hosts pointing viewers toward different sessions they could attend and private meetings they could reserve, providing live commentary in between.

Mr. Bejan said strong engagement and mostly positive feedback from participants gives him confidence that Build and other Microsoft events will be mainly virtual going forward.

Build attendee Debbie O'Brien, of the Spanish island of Mallorca, said the conference was so engaging that she waited until there were breaks in the action to step away from her computer. Ms. O'Brien, who is head of learning for NuxtJS, a platform for building web applications, also described the event as intimate because the speakers were participating from their homes. The format "makes you feel much more like you are part of it," she said.

Salesforce's flagship Dreamforce conference takes over a swath of San Francisco every year. Last November's event featured 171,000 attendees, and speakers including former President Obama. Metallica was a musical guest the previous year.

While Salesforce hasn't announced specific plans for this year's Dreamforce, it has tried to maintain cohesion and keep the pizzazz with the more than 50 virtual conferences it has held this year. An event last month treated participants to a "newscast" by "Saturday Night Live" comedian Colin Jost and an acoustic performance from singer-songwriter Kacey Musgraves, said Karin Flores, the company's head of strategic events.

"The biggest challenge is recreating authentic networking. I don't think anyone has found the silver bullet," Ms. Flores said.

Apple's event has taken on added importance in recent years as it has looked to offset slowing iPhone sales with rising sales of software and services. The event drew 6,000 attendees last year, and the city of San Jose, Calif., estimated the loss of the in-person event cost it $9 million in economic activity.

Apple hopes to maintain the conference's sizzle online. It brought film crews to its corporate campus in early June to record videos of its engineers' training sessions, according to people familiar with its preparations. Apple declined to comment on the videos but has said publicly it is offering virtual, one-on-one sessions for developers with its engineers. It eliminated its fee for participants, which was $1,599 last year.

The company also redesigned an online forum for developers in hopes it can create a virtual version of what Smile Software founder Greg Scown calls "the hallway track," the in-person conversations attendees have bumping into each other.

Apple usually hosts a software showcase on the first day of its Worldwide Developers Conference. It is expected to announce its plans to introduce Macs next year with processors designed in-house instead of processors from Intel Corp., according to people familiar with the matter. The transition was earlier reported by Bloomberg.

Not everyone is convinced online is a worthy substitute for an in-person gathering. Facebook Inc. and Alphabet Inc.'s Google canceled large annual events aimed at software developers, F8 and I/O, respectively.

Conferences are significant sales drivers for the tech industry's biggest companies. Even though going virtual can be less costly than the tens of millions of dollars it can cost to produce an in-person event, "what you lose in potential sales, relationships and seeding future growth is a gut punch to these companies' top line," said Dan Ives, an analyst at investment firm Wedbush Securities. "I view this more as temporary than the new norm."

Okta Inc., a provider of workplace software, changed its Oktane conference in April to a virtual one, scrapping plans for a slide show across three 40-foot screens costing about $100,000 each, among other extravagances. Chief Executive Todd McKinnon spoke from his home for the keynote presentation, which included shots of his children and of his family singing "Here Comes the Sun" by the Beatles.

Mr. McKinnon said the conference generated more than 5,000 sales leads for Okta, compared with just a few hundred last year. "It turned out there were big advantages" to going virtual, he said.

Write to Sarah E. Needleman at sarah.needleman@wsj.com and Tripp Mickle at Tripp.Mickle@wsj.com