Either don't get a new one, or buy one of the newer PSPs that are being now.
But really? Are there that many existing PSP games that are worth keeping? I struggled to find a single game I actually liked enough to play longer than 10 minutes, let alone 15.
I know that FIFA and maybe Ace Combat will see new versions that will probably be better than the existing ones. I also know that from my experience with Playstation 1, 2 and 3 that the desire to play old generation games disappears rapidly when you see the new versions of the games you actually like. Doesn't matter if the graphics and so forth aren't going to be better, usually it's the gameplay updates that make the new versions desirable.
The real loss from losing UMD are the movies. These were excellent value if you bothered to shop around. As a portable entertainment device while traveling, the PSP is better than anything out there. The quality of the screen and sound is superb, and most decent movies can be bought online for around $5-$10.
Hopefully they'll do a much better job of providing downloadable movies, but the cost will nearly double if what Apple has done is anything to go by.
The only advantage I can see to ditching UMD is the battery life will be extended. But the thinner models start off with less battery life anyway, but most planes come with power charging sockets on trips that take longer than 5-6 hours. They provide them for people who bring laptops.
The 24 UMD movies I own cost me less than $200 and I know I won't be replacing my existing PSP just for the convenience of flash ram. I might get both, but I'll be keeping my existing model just for the movies.