It all depends..
When you get into 40 gal pots for 5 gal batches you end up not being able to measure the liquid because it's litterally 1-2 inches off the bottom of your pot.
Depending on your pot, you may also have issues with the pot being too wide and the grain bed being less than an inch or so thick.
There are other similar issues to contend with; but you should usually buy the appropriate pot for your projected batch sizes. If you're slowly buying pots as you go to upgrade then continue using your old system until you have all your pots.
You could use a 40gal pot as a boil kettle though. No major issues doing that. Possibly even do multiple mash(s) and run them off into the larger pot.. Hope this helps