We decided to go with different support/replacement terms to save some money on our renewals for HA. The primary unit has 24x7 with best available time for replacement, the secondary units have 8x5 with slower hardware replacement terms- standalone devices have 24x7. They both have the same software/fortiguard/etc entitlement so there's no concern with having out of date signatures or fortiguard webfiltering breaking during a failover.
For what it's worth (as well), I've worked in budget constrained environments in the past and there were some loopholes you could jump through to minimize what you needed to buy. Also- typically in those environments you don't end up with HA because it literally doubles the costs of everything. I won't go into details, but here are some of the things that might happen in your situation.
If you have a failover, your secondary unit will likely have out of date IPS/AV signatures but IPS/AV will continue to work just with old sigs. If you are running in Active/Active- this may actually already be happening which could be a problem if scanned traffic gets sent to the secondary unit, an attack may not be detected if the signatures are out of date on it. If you have fortiguard webfiltering turned on- any policies using it will begin to block traffic as it can't determine a category. You can't switch fortiguard entitlements willy-nilly so you can't just switch the license from primary to secondary during a failover- typically it's only for RMA replacements. I had to physically move and restore configs on two (thankfully identical) devices that had their support terms swapped by mistake- they wouldn't do it after they were assigned to hardware. If you have a problem on your secondary unit, you will need to buy support for that one. Contracts are retroactive until the date of last coverage or 6 months, whichever is less. So if your secondary device dies and you buy a 1 year contract for it after having been inactive for 2 years- you end up with only 6 months coverage remaining right away. They instituted that policy years ago to prevent people from only buying contracts when they have RMA needs and abusing it.