Hi Locked,
You can register an existing lower as a SBR by completing a form 1 and paying the associated $200 tax. Once you receive your approved form 1 with stamp you are good to move forward with your build.
The temporary change rules are somewhat of a grey area to me so I will wait for someone more knowledgeable than me to answer. I would also suggest consulting an attorney in your area familiar with NFA law. The short answer is yes, you are allowed to make temporary changes to the SBR configuration as long as you can return the rifle to the registered configuration.
The answer to your third question is found here
https://www.atf.gov/firearms/qa/may-...urposes-intent
SBR's are a lot of fun, especially in .300 blackout. Enjoy!