You cannot directly upgrade an prevous vSphere Agent version to vSphere Recovery Agent 8.40. However, you can re-register VRA 8.40 with a vault as a version 7.3x vSphere Agent. You can then run backup jobs from the version 7.3x Agent without reseeding and restore VMs from existing backups. See Re-register a vSphere Recovery Agent as an existing vSphere Agent.
Note: The first backup after a re-registration will not reseed, but the backup could take longer than a normal delta backup. The VRA cannot use Changed Block Tracking (CBT) for the first backup, and has to read all VM data.
The vCenter must be upgraded to version 6.5 before any hosts are upgraded.