Can you change MOS after basic to 18x/11b?

First, you'll have to figure out which you want to do because they're nothing alike.

There's no such thing as 18X, it's an enlistment program, not an MOS. It's only available to initial enlisting applicants. You can apply to SF, go to the selection, and if you're selected you go to the Qualification Course. It takes a year or two depending upon the 18 series MOS you're given.

If you want to be 11B there are two ways. First one, you get to your unit, go to Ranger School and pass. Come back, and it's a quick request through your Career Counselor. Second way, after you've been on station a year, you see your Career Counselor, get an 11B class date, and ship out to OSUT whenever the class date is. That could be anywhere from 3 months to 15 months away.

It's not going to happen. Not until you have at least completed your AIT and graduated. You can then put in a 4187 to go to 11B school. The problem is you are a reservist by your profile and while not impossible, your chances are slimmer, a lot slimmer. Your best bet is to finish your current MOS AIT and then go to your home unit and request to go active duty. Then from there request to go 11B, then Airborne. Then get yourself in serious shape and research going SF.

The Reserves do not have 11B positions or Special Forces units. There are a few prior service NG or AD 11B soldiers in the Reserves that are filling 00F (MOS immaterial) slots, but other than maybe a staff position or liaison-type position, they just don't exist in the Reserves (and definitely are not SPC or below positions). The National Guard has 19th and 20th Group Special Forces and a ton of Infantry units if you are looking to stay part-time.

Either way, you'll likely have to finish AIT before you can request a conditional release then re-enlist in either the NG or AD. However, your best bet is to talk to an NG or AD recruiter as soon as possible and see if they have another avenue for your to pursue.