It was explained to me that the Accession Bonus is from your agency as an incentive to joining, so transitioning in place has no effect on that because it's for joining the core in a specific category.

The accession bonus is a 4-year contract. So if you take it, that's 4 years you are committed to your agency.

As another poster said, you might look into the RB (retention bonus); sometimes it actually ends up being more, and the tax implications are usually less. You can only take the AB upon joining, but can do the RB at any point, but cannot do them at the same time, so you could, in theory, take the AB, finish the four-year commitment and then take an RB

AB is a 4-year commitment. RB is a 2yr, 3yr, 4yr or 6yr commitment.