Vintage Synth Repair

As I'm in project of recapping my older devices, I'd like to ask all  
of you more experienced gurus here your opinion:

1. I started with changing electrolytic capacitors in PSU. Do you  
think it would be good to change all electrolytic capacitors in the  
machine (on digital and analog boards)? I want to use the opportunity  
of opening the machine and to avoid possible problems in near future...

2. Is it necessary to change also all bipolar, tantalum, ceramic,  
polyester... capacitors? Do they also deteriorate in time?

3. Is there some age of the device from which would be good to change  
capacitors? 15 - 20 - 25 - 30 and more years?

4. When changing old capacitors with nowadays non-existing values,  of  
course I must follow voltage or use higher value. Concerning capacity  
value should I use the nearest higher value, too? Modern capacitors  
are usually smaller than old ones, so size is not critical. I'm just  
afraid if in certain kind of circuits higher value couldn't be some  
problem...

5. Do you have eventually any other tips & tricks for maintenance of  
older machines? Of course I do cleaning, reseating IC's in sockets,  
cleaning and reseating all connectors, cleaning keyboards... I will  
also change displays in about 10 of my machines for modern LED  
backlighted types. I disassemble keyboards, clean them, lubricate if  
necessary, change key bushings... If necessary, I change  
microswitches... And I'd like to upgrade OS EPROM's in some of my  
instruments.

6. I have also two old Hammond organ (CV model from 1945 and M3 from  
1960). Do you think I should change capacitors (or even more  
components) in preamplifier (CV), amplifier (M3), tonewheel generators  
and vibrato lines? There are specialized firms offering sets of  
capacitors or other components for old Hammond organ.

I will add that all my old machines work well, and I know there's an  
old wisdom "don't touch it if it works", but... What's your opinion? I  
want to change capacitors because I'm nervous that old machines can  
have problems. But is it REALLY necessary to change it when machine  
works well?

Thanks for any answer.

Daniel Forro