modmod

The product (n.), act (vi./vt.), or state (adj.) of altering an existing thing in such a way that said alterations be easily removed and replaced.

(From "Modular Modification")

If you don't want to mod your suitcase into a backpack longterm, then you can modmod it by sewing eyelets onto the suitcase, and cliphooks onto an old set of backpack straps, so the straps can come off and on whenever you want.