Without knowledge of the Felix (new one to me) and what you require from the upgrade, it is hard to give any specific advice. Motors are obviously a tradeoff between size, cost and power. I'm not sure what you mean by "upgrade" in this context.
There is a lot of general information on the wiki [reprap.org]. Regarding voltage I would highlight the following
There is a lot of general information on the wiki [reprap.org]. Regarding voltage I would highlight the following
although 3-7V is probably ok.Quote
For this reason, stepper motors rated for 3-5 V and 1-1.5 A are generally recommended, as these motors will perform near their peak torque with a current-limiting stepper controller (such as a Pololu A4988).