By Neil Hughes

Published: 09:40 AM EST (06:40 AM PST)

Of the 15 million iPads sold by Apple in 2010, between 15 and 20 percent of those early adopters will upgrade to the iPad 2, one analyst believes.

Mike Abramsky with RBC Capital Markets sees up to a fifth of original iPad buyers switching to the iPad 2 over the next 12 months. But most who buy an iPad 2 in 2011 will be new to the platform, and Abramsky expects Apple will sell 28 million of its touchscreen tablet in this calendar year.

He expects those sales to get off to a strong start this weekend, as Apple may sell more than 500,000 of the iPad 2 after it goes on sale today at 5 p.m. That would nearly double the 300,000 first-gen iPads sold on launch day last April.

The launch will also be bolstered by more than 10 times larger retail distribution, at nearly 10,000 stores, including Apple's retail outlets, Best Buy, Verizon Wireless, AT&T, Target and Walmart. The analyst said initial sales could exceed his estimates if available stock is greater than expected on launch weekend.

New buyers likely waited for the second-generation device, looking for upgrades from the initial product and also waiting to gauge the popularity of the tablet form factor, Abramsky believes. He sees the global tablet market growing to sales of 50 million devices in 2011, a number that would give Apple's estimated sales a 56 percent share of the worldwide market.

If Apple does take 56 percent of the market in 2011, that would be a decrease from the 83 percent of shipments Apple represented in 2010. But a new survey revealed this week also showed that most who plan to buy a tablet -- 82 percent -- said they will choose Apple's iPad.