There is a setting in the swOS web admin to boot to RouterOS. I can't recall exactly where it is but it is there as I used it on my CRS328.
License should be there automatically so you won't need to install one. It shouldn't void your warranty either.
I would recommend changing from SwOS to RouterOS. I have had nothing but problems with SwOS. Particularly issues with fans not working at all or spinning all the time and huge issues with buffer overruns. All of which seem to have been resolved by changing to RouterOS.
However, VLANS in RouterOS are still a pain to configure.