To answer your question on replacing tubes... they simply pull out and you can replace them by gently pushing new ones in. Give them a little wiggle as you pull gently, and they should come right out.
If one fails, you don't need to replace all 4, but if you don't then you'll have a mismatch. The new one will operate at a higher level than the old one. You may or may not notice. I would replace in matched pairs, at least.
If it sounds fine, I would wait until one fails. You never know... the system may have only had a few hundred hours of use, and the tubes should last thousands of hours. Don't replace them just because they are old... you may end up spending money for nice NOS tubes when you already own good, usable vintage tubes.
If one fails, you don't need to replace all 4, but if you don't then you'll have a mismatch. The new one will operate at a higher level than the old one. You may or may not notice. I would replace in matched pairs, at least.
If it sounds fine, I would wait until one fails. You never know... the system may have only had a few hundred hours of use, and the tubes should last thousands of hours. Don't replace them just because they are old... you may end up spending money for nice NOS tubes when you already own good, usable vintage tubes.