Mike H.
I have the same issue. 
  1. old license file installed and server started.... works fine.
  2. stop the manager
  3. install new license file
  4. start the manager...... it starts but then stops after 5 seconds.
flexnet log:
10:14:01 (lmgrd) Server's System Date and Time: Wed Jul 26 2023 10:14:01 Eastern Daylight Time 10:14:01 (lmgrd) pid 532 10:14:01 (lmgrd) SLOG: Summary LOG statistics is enabled. 10:14:16 (lmgrd) The TCP port number in the license, 1055, is already in use. 10:14:16 (lmgrd) Possible causes: 10:14:16 (lmgrd)    1) The license server manager (lmgrd) is already running for this license. 10:14:16 (lmgrd)    2) The OS has not "cleared" this port since lmgrd died. 10:14:16 (lmgrd)    3) Another process is using this port number (unlikely). 10:14:16 (lmgrd) Solutions: 10:14:16 (lmgrd)    1) Make sure lmgrd and all vendor daemons for this 10:14:16 (lmgrd)       license are not running. 10:14:16 (lmgrd)    2) You may have to wait for the OS to clear this port. 10:14:16 (lmgrd) Retrying for about 5 more minutes 10:14:34 (lmgrd) Still trying... 10:14:52 (lmgrd) Still trying... 10:15:10 (lmgrd) Still trying...
  1. so I killed the lmgrd processes and restarted the manager... still stops by itself after five seconds.
  2. killed the lmgrd processes again, reinstalled the OLD license file, started the manager...works fine.