Accepted CNA position but got an interview for another position in the same hospital

I accepted a per diem CNA position on a Med Surg/Telemetry Unit at a well known teaching hospital in CA. I intend to work 3 days a week to equate to working "full time".

This morning, I got an email from the same teaching hospital inviting me to interview for a FT CNA position on their Perinatal Unit. I have an interest in becoming an RN and working on L&D, mother baby, postpartum, or NICU so this position would definitely be amazing!! I'm scheduled to interview next Tuesday.

The problem is I already accepted the per diem position so I'm not sure what to do.... I haven't done any paperwork or orientation. My start date is supposed to be April 24th. 

I'll do the interview to see how things go, and hope they offer me the position. If they do, I'd consider retracting my original offer for the per diem position. I know people retract their offer all the time for another offer but since this is the SAME hospital I don't know how to go about it. It feels awkward and like I'm doing them dirty. Would this make me look bad?

I can't work both positions at once (I asked a recruiter). I'd like to work "FT" either way. So my choices would be:
1. Work per diem, 3 shifts per week, flexibility to choose my shifts, earn extra ($29/hr)
2. Work FT on Perinatal, speciality of interest, earn less (their range is $23-31/hr. based on my experience I'd earn maybe $25-26/hr), benefits

Don't know which choice is better...

FYI: I'm starting a 8 week physio class on April 10th and might take a Summer class. I'm 23. I'm planning to start nursing school next Spring in January 2024 (assuming I get in). I'd work per diem during school and limit myself to 1 shift per week. So if I do FT, I'd only be able to work FT for about 8 months and then switch to per diem.

And yes, this is definitely a good problem to have so I have no complaints haha.