Actually there is, in principle, no reason why you couldn't update an older but very successful machine with some of the F35 sensor fusion stuff and obtain something even more effective any time when you don't need stealthiness.
Actually there is.
The Block I F/A-18F Super Hornets have slightly different air frames and shaping than the F/A-18F Block II and later Super Hornets, as well as the EA-18G Growler. This was done to provide a common air frame for the future Super Hornets and Growlers to ease the potential logistics burden by having a single air frame able to accommodate the wiring harness for Super Hornets and Growlers.
To take an older design and do the same thing but make it compatible with the avionics suite of an F-35... That would require a fair amount of redesign work to fit a compatible wiring harness (fibre optic in the F-35 IIRC) as well as the various sensors and avionics. Fitting EODAS for instance, would require that the E/O cameras be fitted to provide all-aspect coverage of the air frame, and yet the air frame could have other components or structural members installed where an EODAS camera would need to be mounted. Unless the nose of the older fighter was a match in terms of shape and size for the F-35, it would be highly unlikely that an APG-81 AESA radar could be fitted. Again using the Super Hornet as an example, the nose of the early Block I Super Hornets can fit an APG-73 radar, but not an APG-79 AESA radar like is fitted on board new Super Hornets and the Growler.
It might be possible for an older fighter design to be modified to take either the senors and avionics systems of an F-35 or and equivalent, it would be a very significant redesign which would take considerable time and resources to accomplish. The end result of that would deliver a 4.5 gen fighter which would like at least approach if not already exceed the purchase price of a full rate production F-35, which would have all the advantages of the redesigned aircraft and then more on top.
If the idea is instead not modify an existing design for new production, but to fit the F-35's systems to existing aircraft already in inventory, then I would have to say that is not possible IMO. Some systems could possibly be fitted to older aircraft, not all the systems would be able to be installed.