If it hurts, do it more often (
https://martinfowler.com/bliki/FrequencyReducesDifficulty.html)
Upgrading a production environment is never risk-free (unless you can afford downtime of course) but neither is not upgrading. 3.7.14 is 2 years old, 3.7 has been out of support since September. It's high time to upgrade.
Yes, it's an in-place upgrade. Starting with 3.7, RabbitMQ has feature flags to allow mix-version clusters (so you can have 3.8 nodes join your 3.7 cluster until all nodes are 3.8).
PS Let us know why it was such a pain to install for you because it doesn't have to be. I installed 5 RabbitMQ clusters today... :)