Do I need new BMC-91 for reinstatement?
Yes if the original BMC-91 has been canceled or non-renewed. The authority can't be reinstated until active BMC-91 (or BMC-91X) coverage is on file with FMCSA, so new insurance has to be bound and the insurer has to file it with FMCSA before the reinstatement can go through.
49 CFR Part 387 requires active BMC-91 insurance for any operating authority. When the authority is suspended or revoked for insurance lapse, the underlying insurance has to be replaced before reinstatement.
The flow: the carrier secures commercial-auto liability coverage from a new (or returning) insurer. The insurer files the BMC-91 or BMC-91X electronically with FMCSA. Once the filing is in FMCSA's records, your reinstatement is submitted to FMCSA promptly. FMCSA controls the timing of its review.
For carriers reinstating after a non-insurance suspension (audit failure, out-of-service order), the original BMC-91 may still be valid if the policy was kept current. Check FMCSA's Motus public search (by USDOT number or company name) to confirm the insurance filing is still active before relying on it; have the insurer refile if it lapsed.
BMC-91 and BMC-91X are both certificates of liability insurance that the insurer files with FMCSA; which one your insurer files depends on how the coverage is written. Either works as long as it meets the minimums in 49 CFR 387.9.