After a long time we decided to try "second servis" upgrade from pfSense 2.3.5-p2 to 2.4.4-p3 on our remote offices. Everything went fine, so there is a little survey:
OpenVPN site-to-site (shared key) tunnel has so called "dynamic" gateway in 2.4.x on client side, which is created automatically on the system startup. So if your old version has a manually created VPN gateway (routes to headquarter not included in OpenVPN config...), you have to remove this gateway before upgrade. My best practice was backup old configuration, upgrade, login to the upgraded pfSense and completely remove the old OpenVPN client and his TUN interface. Then I created new OpenVPN client. VPN gateway was created by system and a I could set up required routes again.