If I haven’t shipped off to basic yet, could I change my MOS from 14P to 11x?

Yes, renegotiating before you ship is possible, and 11X is usually available if you are qualified. As mentioned already, what ever package of incentives that came with 14P will be voided, and whatever incentives that you qualify for with 11X will be available to you. Talk to your recruiter.

Talk to your recruiter- it may or may not be possible. But if it is possible understand this- if you received a bonus of any kind it will be removed nor will you get a bonus with the new job. This may or may not matter to you but it is something to consider.
Another thought would be to do the 14P job and then come re-enlistment time request to go 11X.

If you want to be an Infantryman, don't settle for "you're basically Infantry, bro" and end up regretting doing what you train to do. Talk to your Recruiter and see what you can do about changing your MOS, but you might end up not being able to get it and having to ship off for the 14P.