Take a mental step back and think about this logically for a moment, ask yourself, why would it be not so smart to switch brands?
Such a notion seems like a paranoid delusion, 1mg finasteride is 1mg finasteride all the same, regardless if it's brand name Propecia or generic Fincar, etc. The only difference between brands would be the colour/shape of the tablet and the binders, fillers and excipients used in their manufacture. All of which have no bearing on the drug's efficacy.
Absorption and concentration shouldn't differ between brands. If you're worried, just make sure the brand of finasteride you buy is manufactured by a pharmaceutical company that has to adhere to certain standards (look for British, US, EU or Japanese Pharmacopeia on the package label)