They all need new wings, longerons etc. The USAF held an open day to get some industry estimates of costs - but it looks unaffordable. See here.
https://www.fool.com/investing/2016/...s-project.aspx
"According to data from FlightGlobal's World Air Forces 2016 report, the U.S. Air Force currently has 416 F-15 fighters (models C through E) in its inventory. More than 180 of these are recent-model F-15Es. But Defense One reports that 235 of the planes are older F-15C and D models.
Now here's the problem: The basic F-15 design has been flying since the 1970s, and some of these C- and D-variant warbirds are getting a bit long in the beak, and need to be either upgraded or retired. To keep them flying for their full hoped-for life spans (which won't run out for another 25 years), the Air Force says it must spend at least $12 billion (Defense One estimates the cost at "tens of billions of dollars") to refurbish the F-15 fleet with upgraded electronics, new wings, bigger fuel tanks, and other structural replacements.
Even these upgrades won't succeed in transforming a fourth-generation F-15 into a stealthy fifth-generation combat jet, of course. But they'd increase the plane's range, endurance, lethality -- and life span. And over at Boeing, they're all for the idea. Steve Parker, Boeing's vice president of F-15 programs, contends that teaming up upgraded F-15s with stealthy fifth-generation F-22 fighters from Lockheed Martin would enable the Air Force to put a lot of metal in the air, and ensure air superiority "into the 2040s."
But at what cost?
At the low end of Defense One's estimated "tens of billions of dollars," $20 billion spent on upgrading 235 F-15C and D models implies an upgrades cost of more than $85 million per plane....... Looked at that way, the prospects of getting the Air Force to pay for upgrading the F-15 fleet -- or getting Congress to approve the funds -- seem pretty dim. The same money that would be needed to upgrade 235 older-model F-15s could instead be spent buying 235 brand-new F-35s from Lockheed........
A second alternative suggested by Defense One -- and this is an idea the Navy has been mooting as well -- might be to take the money the Pentagon would otherwise spend on F-15 upgrades and invest it into developing a sixth-generation jet. Tentatively titled the "Penetrating Counterair" fighter by some, and "F-X" by others, such a 6G jet couldn't possibly be ready to enter service before 2030......."