The sound you describe is probably not going to change very much. Ias I have written here and on the old forum site before, an audible increase in diaphragm tension (my speculation) took place in the early-to-mid 2000's.
That increase will make it more difficult that the tension, and with it the sound timbre of the capsule, will be affected by high SPL-or other types of abuse.
As the Mylar® material used as substrate for the diaphragms of K87 and K870 does not deteriorate much with age, and because the diaphragm tension is now above the point where it is easily affected (lowered) by environmental influences, your capsule will probably continue to sound the way it does for a long period of time.