I'm not sure if it's valid for all OW airlines, but I think you should keep in mind that some (most???) OW carriers only allow you to upgrade using their own miles (e.g. you can only upgrade a flight on QF metal using QF Frequent Flyer miles).

That's at least what I was told by both BA's and AA's representatives when I tried to upgrade flights on BA/AA using my QF miles. They said it is not possible.

If that's confirmed, then you should join MA's FFP in order to be able to upgrade your MA flights.

The more expert FT's will be able to confirm or dismiss this, I'm sure.