Have you read the rules and regs for BEAR Program participation?
In general, when you complete your retraining, you must re-enlist/extend in order to meet the SRR requirements you agreed to when you were accepted into the BEAR Program. Failure to do so will invalidate your bonus. Yes you have 90 days following graduatiing to re-enlist.
6–10. Service remaining requirementsand extensions
a. All soldiers approved for entry into the BEAR Program must extend their enlistment a sufficient amount of time to ensure they will have 24 months time-in-service remaining as computed from the completion date of training.
b. An SRB is paid only for “additional obligated service” and any time remaining on the extension of enlistment that is executed upon entrance into the BEAR Program is considered “previously obligated service.” An SRB will not be paid for any unserved time remaining on the extension of enlistment at the time of reenlistment following successful completion of retraining into the new PMOS. Soldiers participating in the BEAR Program may be forgiven up to 24 months of “previously obligated extended service” provided they reenlist (Option E-1 only) within 90 days after completion of training. Implementing instructions will be announced via RETAIN and/or MILPER messages.
c. Extensions to participate in the BEAR Program are accomplished as a condition of acceptance into the program.
Cancellation of the extension is not authorized for soldiers who voluntarily withdraw from the program or who are involuntarily removed from the program for any reason (that is, academic failure, punishment under UCMJ). The soldier is considered, in either case, to have received the benefit of the extension.
As for promotion, the reg (AR 600-8-19, Ch. 3-6) states that:
e. Special MOS alignment promotion. Soldiers with approved reenlistment/reclassification in MOS designated by HRC for special MOS alignment promotion to meet Army readiness will be promoted effective the graduation date of the MOS-qualifying, which occurs after all training phases have been completed and the MOS has been awarded.
Soldiers who fail to complete training as scheduled, become disqualified for award of the new MOS are no longer eligible for promotion under this paragraph. The HRC memorandum and this paragraph will be cited on the promotion instrument as authority.
You should always understand ALL the requirements and benefits associated with career altering programs.