If you will delete enough songs to get your intial library down below 25,000 songs it will start. After that it has either matched or uploaded your songs, highlight the uploaded and see how many that is. The uploaded songs are the ONLY ones that count towards your 25,000. But you have to get below 25,000 to get it started. Please see my other post on has the 25,000 limit changed with explicit steps. After you get it started and it has finished the initial steps then you can start adding your music back to your library until you have reached the 25,000 uploaded song maximum. You most likely don't have near as many to upload as you think. But you have to get the library below 25,000 to start and then add back after each time itunes match completes.
My library was really large and I deleted enough to get started and then started adding back and i'm no where near the 25,000 downloaded songs that is the limit. Most of my library was matched and therefore it didn't count towards the 25,000.
1. Start by downsizing your library enough to get started.
2. Let match do it's thing and finish with what you have in there.(This may take awhile so don't quit. Just walk away from it and let it do it's thing for however long it takes. Don't turn it on in your device yet.)
3. After match has finished, check how many had to be uploaded and that will count against the 25,000.
4. Start adding back into your library bit by bit and go to store, update match and let it do it's thing again.
5. Keep repeating the process until your whole library is back.
6. Now turn it on in your devices (ipod, iphone, ipad)
I'm sure you won't hit the 25,000 uploaded songs.