Important date to look into, is what happens in the 3rd season. Will a longer deal be negotiated (make him a restricted free-agent) or go through until the fourth year, which would make him an unrestricted free-agent afterwards.
Yes he's an UFA after 4 years.
http://www.bostonglobe.com/sports/2015/07/20/celtics-sign-jordan-mickey-four-year-million-deal/PoW5bSyaA4oLJDbhfYs4WN/story.html?s_campaign=bostonglobe%3Asocialflow%3Atwitter
The two guaranteed years on the deal — one of the largest ever given to a second-round pick — will total about $2.4 million, a source said. That will be more than some late first-round picks receive through the league-mandated rookie scale. Also, Mickey could become an unrestricted free agent after four seasons.
Not familiar enough with the CBA to know how he can be "re-made" into a RFA as you described. Any source for your claim?
If the fourth year is a team option rather than an unguaranteed year, the Celtics would have the same situation that the Rockets had with Chandler Parsons: decline the option and make him a restricted free agent or exercise the option and let him become an unrestricted free agent in a year.
For further confusion, you can actually have a team option and an unguaranteed year in the same year of the contract. So year 4 could be a team option, which, if declined, would result in RFA. But if taken up, that year of the contract could still be unguaranteed. Don't know if this is the case with Mickey, but the Rockets have done that a few times before, including, I believe, with Parsons.
Update:
According to Spotrac, it is indeed a team option/non-guaranteed year for the final season.
Year 1: $1,170,960 (fully guaranteed)
Year 2: $1,223,653 (fully guaranteed)
Year 3: $1,276,346 (fully guaranteed effective 7/1/17)
Year 4: $1,329,039 (Team option. If team option is picked up, contract is still non-guaranteed, until 7/1/18, at which point it guarantees fully).
So if Mickey plays well for three years, the C's can decline his option and make him a restricted free agent, and therefore easier to keep since they'll have full Bird rights. If they decide they want him at his low year 4 salary, they can pick up the option. They will still have full Bird rights, but he will not be a restricted free agent. (Note: This assumes that the rules regarding restricted free agency do not change in any potential CBA renegotiation before the 2017-2018 season). However, if they decide they don't want to keep him after year three, they can pick up the option, and try to trade him on draft night, either to a team that's interested in him, or as salary filler that can be waived. If no trades are available, he can still be released before July 1st of that year with the Celtics owing him nothing.
TL;DR -- If Mickey seems like a stud, team can make him RFA after year 3 to lock him up long-term. If he's useful, but not a long-term keeper, team can keep him for cheap in year 4.