Most things in 3.4 are actually fully compatible to older releases. The new S2 geo indexes are an exception here. They are a completely new implementation and use a different storage format. However, index data will be automatically be converted into the new format when upgrading from older releases (e.g. 3.3) to 3.4.
To be on the safe side, it is a good idea to consult the list of incompatible changes/changed behavior before upgrading:
https://github.com/arangodb/arangodb/blob/3.4/Documentation/...
That may seem like a huge list at first, but many items on it should actually be minorities.