Hi, i'm a beginner and i switched to holding the guitar in a classical position while practising and i am finding it very comfortable and easier to play but after switching to classical position i am experiencing wrist pain and was wondering if i should make any changes to prevent wrist injury and i play with my thumb behind the neck most of the time.
One more thing i need help with is playing basic chords with thumb behind the neck of the guitar because i find it kind of hard to play chords that way rather than thumb over the neck, which one will help me in the longer run?

Your thumb should almost always be behind the neck. Tilt the guitar up a bit so the headstock is at about the same level as your shoulder. Keep your wrist as straight as possible while still maintaining a good arch with your fingers.
Quote by Geldin
Junior's usually at least a little terse, but he knows his stuff. I've always read his posts in a grouchy grandfather voice, a grouchy grandfather with a huge stiffy for alternate picking.
Besides that, he's right this time. As usual.