When to cut back herbs

I've got a few clumps of oregano and sage that were getting a bit leggy last year and sparse in the centre. I could replace them with new plants but I thought I might try cutting them back hard to see if they'd regrow from the base and fill out again. Can anyone tell me the best time to do this? Can I do it now or best to wait till the frost has finished? I've got no choice but to replace the big clump of kitchen thyme because my evil lurcher dug the whole lot up.

I've go a straggly lemon tyme in a 10 inch pot it's probably 3 years old.

Should change its compost or prune it back hard?