It’s honestly up to you.
If you’ve closed your current corset and it’s starting to feel loose, then I’d suggest getting a new one if your wallet agrees.
What I like to do after closing a corset, is wear it for about another month or 2, see where I am in my corset training progress, and then go from there.
I may keep the closed corset as a “maintanence” corset, and get a new corset to continue my waist training to reduce my waist even more.