I cannot improve on Jongware’s comments re moving from a PPC Mac to an Intel Mac as I have not faced that problem.
But on my iMac (Intel, late 2007 24-inch model) I have InDesign CS, CS2, CS3 and CS4 (all installed in chronological order) as, for a variety of reasons, I need to open files in each of those formats and, after working on them, be able to save them in their respective formats. InDesgn CS is rarely opened these days (and, indeed, Adobe tells me I shouldn’t be able to open it — but I can) but each of the others functions without drama.
In these circumstances, life is made a lot simpler by using Rorohiko’s marvelous little utility, Soxy. Other than being a happy customer, I have no connection with the company. Soxy intervenes whenever I double-click on an InDesign icon, recognises which version created the document and launches that version of the application so that it can open the file. Too easy!