I recently bought an acoustic guitar, and it had this beautiful action on it. I've restrung it recently (same gauge strings) and now there's a very noticeable difference in playability. Is this normal, am I going to have to get the action adjusted already?
Usually when you restring an acoustic guitar, it will feel, and play, slightly different for a while. New strings that are fresh on the guitar tend to feel a little firmer and heavier to the fingers, but as time goes by they will start to feel a little less tense as they begin to stretch.
Are you absolutely sure that they are the same gauge strings? The reason I ask is because if they were heavier than the strings the guitar came with, the increase in tension (heavier strings cause more tension on the neck) could cause the neck to bend slightly inwards, causing an 'up bow,' and therefore increasing the gap between the fretboard and the strings, giving a higher action. It would be a very small change, but still noticeable to you as you play it.
There are other things that could have caused a change in feel. If you started to store the guitar somewhere different to where you were before, a change in temperature or air moisture could cause the guitar to expand or contract slightly, potentially affecting the action. I do however suspect that this is not the case as this happens over a longer period of time.
The bridge piece may have fallen out of it's correct seating when you changed the strings. It may have risen up or down between the old strings being taken off and the new ones put on. This would have a noticeable effect on the action.
If the guitar however sounds different after a restring, that is, sounds out of tune (even when tuned according to a tuner) you probably have intonation problems. See this previous question about acoustic guitar intonation adjustments.
I recommend to most people that after they purchase a guitar they ask the seller to set it up for them. Many guitar shops offer a free set-up for each guitar purchase, but sadly if you are unlucky then yours may not. The question and answer I linked to outlines roughly what the technician will do. A professional set up will ensure that the instrument is in the best condition possible for playing.
Hope this helps.