Iran has done alot when it comes to maintenance and modernization of its various equipment. However there is still more work to be done obviously and progress is ongoing.
Before 1979 Iran bought weapons exclusively from the west. After 1979 Iran for a time couldn't buy weapons from anyone, then had to buy a combination of whatever they could get their hands on, especially in the 80's during the Iran-Iraq war.
As a result Iran has far too many different kinds of fighter jets in inventory and far too many kind of tanks for example. Despite these issues Iran's military are extremely resourceful and have done a good job keeping everything in order.
In the coming 2 decades some older tanks and infantry vehicles must be phased out and especially fighter jets like the F-4, they have to be phased out.
In many sectors Iran is mostly self sufficient. When it comes to ground vehicles, or air defenses for example, Iran seems to be more or less good on its own. Fighter jets, Iran has some options, however Iran's military leaders are picky and want to make sure they get their hands on a solid platform. Iran's military are more concerned about purchasing a proven platform and getting value out of their purchase rather than simply investing in hype.
For example, Iran could have purchased the S-400 instead of S-300. If you recall the initial deal was delayed for a very long time. The issue is that when it comes to the S-400, the Russians do not give out the source codes.
Turkey for example does not have the sources codes for the S-400 and some maintenance has to be done by Russian technicians. The latest export variant of the S-300 Iran ended up purchasing is a different story. Iran has the source codes and can do all the maintenance on their own.
Yes the S-400 export variant is slightly better than the last S-300 export variant but we saw how under Israeli pressure the Russians would not even give the Syrians the source codes for the S-300, which ended up being a stumbling block and headache for Syria.
At the end of the day, self sufficiency and depending on proven, cost effective technology are key factors for Iran's military.
Nah dude, just NAH.
Upgrade all your T-72S to Karrar standard (1 chassis).
Upgrade all your BMP-2 with Iranian produced Kornet and (murrican inspired MK 19 Mod 3) 40x53mm AGL and space and cage armor to BMP-2M standard (there are several "M"-variants).
Upgrade all your already upgraded BTR-60PPB to a more modern standard, best would be exactly modernized BMP-2 turret and give them a new 8x8 gun variant - 105mm turrets, like the Cubans gave their 100mm turrets from T-54B/T-55A (a bit modified).
For mobile artillery, use only 8x8 or 10x10 chassis, armored, 12.7x108mm RCWS.
Just use FEWER chassis, much easier to produce, to maintain, a lot cheaper even if the products are from higher quality.
Greetings!