Theme's for jams are generally to BOOST creativity, not to cut it down. So for a theme to be announced on short notice, it will make you design a game from scratch including concept. This means you have a lot of free reign in your design, instead of baking/nailing down every detail WEEKS before (which DOES cut down creativity).
In fact, you could even start developing the game then already, that's why the theme is always announced a little before the kick-off, so everybody has exactly the same amount of time and work in their projects.
All in all, I think you can still make one of the games you already designed, since theme isn't the only thing games are rated on. In fact, it's only 1 of 6-7 things the game is rated on. If you still make a good game with the limitations presented, theme shouldn't be a problem.
And also, 2 weeks is more than enough to write/design/develop a proper game of the expected quality.
That's how I feel anyway.