Do I really need Desoldering for replacing resistors/caps?

i don't know the age of the unit you're talking about but if it's older than 5 years it'll have lead solder, and now (certainly here in the UK) lead solder is a rarity, the lead-free stuff doesn't play so well with leaded, so if you did decide to do it, and the unit is of an age where it's leaded solder, best to remove as much of it as possible ... you'll still have a surface covering and thats ok, but mixing a blob of one with a blob of another is not so good

just my 2pennyworth, and my first post here having lurked for some time but only recently signed up

cheers guys

Jim

Thank you terry9 and Jim.

I do it different. Avoid de-soldering if possible. I sacrifice the bad component. Cut it apart with diagonals until interior leads are accessible. Grab one lead with forceps. Apply just enough heat to extract the lead. Do the other leads the same way - individually. Don't rock it out. Put a probe at the solder filled hole. Apply heat until the probe slides in (sorry). Allow the solder to set before removing probe. Insert new leads. Apply heat and add a little fresh solder until it all flows; and, Bob’s your uncle.

Older solder oxidizes and stops wetting leads/pads.  In order to obtain good solder joint you need flux.  Flux serves two purposes - it cleans surface and prevents oxidation. Flux can be delivered with new solder containing flux or flux syringe.  If you don't have flux syringe you have to add substantial amount of new solder ending up in excessive amount of solder.  That's why you have to remove existing solder first  (In process of replacing SMT parts old solder is often left on the pads but then new flux from syringe is added before soldering).  Yes, you can make it easier by cutting leads of old components, but you have to remove old solder.

Thank you very much for your advise.