There's 9mm available and MAYBE 5.45 coming. You can change it yourself, but you must disassemble the rifle.
The ACR has a quick change barrel in theory, but not in practice. Rebarrelling ACRs typically mean scavenging the gas block/quick change components from the 16" barrel. So you have to choose. I haven't seen different barrel lengths or calibres available anywhere really, and with bushmaster increasingly looking like they're going to scrap the line entirely, I wouldn't hold my breath for any either.
Sacrificing the gas system from the 16 inch factory barrel is a no brainer if you're going with a non restricted conversion. Once the rifle has been reclassified and is legally non restricted it would be illegal to reinstall the 16 inch without re-registering it as restricted, even if you did it at the gun range where you are allowed to shoot restricted firearms. As soon as you put the 16 inch back on you are in possession of an unregistered restricted firearm and you have 30 days to register it as such, during that 30 days you are not allowed to fire it, only to take it to a gunsmith or certified firearms verifier.
I spoke directly with the RCMP firearms lab to get that info so don't bother arguing your opinion on it.
If you're going with another caliber but remaining restricted things change a little since the gas system isn't cheap.
I have 3 non restricted conversions for my ACR, 223, 300BLK, and 6.8SPC, I have 2 complete gas systems (minus the regulator which I only have one of) and while it is not convenient to swap the gas system it is not hard and doesn't take more than a couple minutes.
Parts are out there but a little slow on delivery and a little pricey. To me it's worth it though, in my opinion the ACR is the best option we have available to us in the $3000 range non restricted rifle department. A used rifle can be bought and converted for under $3000 and if you search the EE and wait for the right deal they can be found for around the same. A buddy of mine just picked one up already converted for well under $3000.
I like the Tavor but it isn't a rifle I feel I need to own.
I've owned a Swiss Arms Classic green target, a couple HK SL8's, a mini-14, numerous AR's, and still own an AR-180B-2, three AR-15's, and an AR-10 so I've spent time with most of the options we have available to us and the ACR is my favorite of the non restricted options, I've also spent numerous days playing with XCR's and a few Tavor's, and a T97. If the AR was non restricted I'm not sure what I would own besides those but whatever remained would be just to have something different.
Sure the ACR is a little on the heavy side but I can still ring the gong at 100yds standing freehand, it's weight contributes to how smooth the rifle is to shoot and if you keep your optics light it isn't very noticeable at all. All of the modern piston driven options need to go on a diet.
My favorite rifle for hauling around in the bush and just everyday plinking is my AR-180, it's so light and smooth shooting, it's no sniper rifle but it will keep up with a Tavor or XCR any day.
I think all the rifles we've been arguing about in this thread are good quality well built rifles (except the T97) and they all have their advantages and disadvantages, it's up to each of us to decide which one fits our personal needs individually and buy the one we like the best. There is no one best non restricted rifle for every application unless we could have AR's in the bush. The Tavor is not for me but I don't look down on someone for owning one. Whatever gets you out shooting and enjoying our sport is the right rifle for you.