I am currently playing a ranger beastmaster but am having a case of buyer's remorse (really missing that level 6 feat). Instead of changing class, is it possible to change to another ranger archetype or maybe just the basic ranger class?
Yes if your GM allow it. Strong roleplaying reasons would help a lot your argument. Paizo even included it as an option on some specific cases. Ex: the heretic archetype for Inquisitor.