I'm not sure about the upgrade path requirements but I do know you should think very carefully about what you intend to use your license for. Only go for the full PPL if you're adamant you will be going professional within a few years, or will be going for your IMC or IR rating.
I only have the NPPL and it does me just fine. I rarely find myself grounded through weather and have never been weathered in away from base. You can add ratings for tailwheel, retracts, VP prop, etc to the NPPL and fly anything up to a Rockwell Commander 114 on it so you're not exactly heavily restricted with an NPPL.