Jhm, get the replacements but don't throw out the old carts. If something bad happens and the pool becomes a swamp, you can use the old ones for the 'heavy cleaning' without prematurely shortening the life of the new carts.

I believe that one can get a longer life out of carts by cleaning them when the pressure gauge rises 5 PSI from clean (as opposed to the often recommended 8- 10 PSI rise) - the 'stuff' doesn't become as embedded and therefore, washes out more thoroughly and easily