Time Machine - dilemma in partitioning external hard drive

I have been using Time machine to back up my Macbook (80GB hard-drive) onto a 500GB external hard drive. I had originally partitioned the drive into A = 150 GB, and B = 350 GB. Time Machine was backing up to Partition A.

Unfortunately, Partition A is out of space.

I would like to have Time Machine start backing up to Partition B instead of Partition A. So ideally, Time machine would back up onto B and I'd have A available for other items.

However, Partition B requires reformatting before this will be possible (and this would erase everything already saved onto partition B). Is there an easy solution to this that doesn't require buying a new external hard drive?

Thanks so much for your help!

 

I would keep Time machine on partition A except that I don't want to lose old information (as it deletes the oldest copy with each successive backup).