If you are on-premise, you can keep Odoo installed forever with no live databases. Then, if needed, you can restore your legacy Odooe14 database and the 30-day timer will begin. But this is a very bad idea because eventually, your system won't be available. This works great with virtual machines as well - but again, eventually they won't work just because they get obsoleted.
Far better is for you to do your full year-end, close all of your books, ensure everything is finished and then export it all to .csv. You should also do all year-end procedures and reporting so you have a full record of "everything".
Your entire accounting history is in your GL so having it as a .csv gives you everything you'll need in an emergency.
But your best option is to ensure that your year-end is done perfectly and then begin on your new accounting system.
Remember, you never "migrate" an accounting system; you close your books correctly and then open new books in your new system using the year-end totals. History then becomes a matter of searching your year-end reports and .csv files.