It's exhaust pressure compared to the intake pressure, so 1.8 EPR "10 years later" will give you the same thrust level. But the EGT, N1, N2 and N3 could be slightly higher due to wear and tear. There are maintenance parameters, for example, of how deep the leading edges of fan blades can be filed off for nicks and cracks. So, N1 [fan] speed alone is not as an accurate measure of thrust as is EPR, . . . as the blades will move less air when its leading edges are filed.