A commission is a commission. It doesn't really matter which of the 7 uniformed services it is in, all commissions are equal (other than rated vs. non-rated, warrant vs. regular commissioned, line vs. non-line, and LDO's vs. non-LDO's, of course...). If you can convince the service to take you and you have no break in service, then you will come right over.
It is very much possible to switch between branches with a commission (similar to the enlisted side), but it takes time and patience,
and a lot of planning.
I assume you mean Army National Guard when you say "National Guard", so in that case-- talk to a Guard recruiter. To go through ROTC as a Guardsmen, you will nearly always need to be actively enlisted in the Guard. Both Army and Air technically let you choose AD or ANG/ARNG at the end of your ROTC experience, but again-- talk to a recruiter first and be open about it. From what I've seen, the Army has a
lot of Guardsmen that do that. ANG typically pays for the school and then sends the graduates to TFOT/OTS.
Lastly, if you go through any branch's ROTC you will almost always have to serve your initial commitment in that branch (I know of one guy that did AFROTC and then commissioned into AROTC his last semester, but that was a unique case).
@kinnem is right that you shouldn't make it Plan A.
Anything is technically possible in the service, but it's always best to just be normal and go about things the normal, regular, expected way.
@CitadelN88 is also dead on the money that the longer you are in, the harder transfers are to get. You have to keep in mind you will still need to do each service's technical training, will need to show proper career progression, go to the professional military education courses at the appropriate rank and time, etc.
Honestly, the only "easy" branch transfers I see are AD to Guard, and that is only when it's Army to ARNG or AF to ANG. Even then, it can be a little tricky.
Lot of information there, so...
TL;DR: If you want to be in the Navy, then join the NROTC. If you want to be in the ARNG, then do AROTC. If you want to really gamble, then do neither service's ROTC and apply to Navy OTC after you graduate. You can transfer your commission between branches, but it is not super easy and trying to transfer an ARNG commission to an AD Navy commission will be even harder (a Navy Reserve commission will probably be fairly easy). Either way, you will almost always impact your ultimate career progression when you switch branches.