How? That is good question, but Sony announced plans to do so in Japan
It is anyone quess, on how it will happen, most say trade-in via selected retailers to get coupon code to download the game from PSN at reduced prices
Well reports are coming on the 'net today that Sony execs. had a meeting with retailers in Japan and they told them their is plans to support previous owners with UMD discs once they make the jump and purchased a new shiny PS-Vita that only supports "cartridge games" along with "digital downloads"
I really doubt this will happen, Sony say the same thing with the PSP-GO before and even after launch, and nothing ever happen, and still has not, if this "trade-in" program finally does appear, will they be kind enough to let the PSP-GO owners stuck with have an out-dated machine take part in this program also finally.
And second what about this 20mb download limit that been confirmed for 3G on the PS Vita, so you be forced to do the d/l via USB on the PC or Wi-Fi then instead?
And thirdly, this might work in Japan with retailers, but come to North America, most retailers would turned their noses up at you, unless you have find your original paper slip of purchase sale, and how many of those for those UMD discs you have still are also intact and readable.
NEWS SOURCES: Geek.Pikimal and EuroGamer