Replacing it with one of the same rating means that you don't need to look at upgrading the wiring.
When you ask can you do it yourself, I presume you mean the Part P stuff?
Regulation 12(6A) sets out electrical installation work that is notifiable. All other electrical installation work is not notifiable – namely additions and alterations to existing installations outside special locations, and replacements (like for like), repairs and maintenance anywhere (this includes special locations such as saunas, swimming pools and within the area of special location in a bathroom).
So you'd be OK doing like-for-like replacement.
Regarding pipework:
You just have to deal with it as best you can, and the best solution will be highly situation-dependant.
De-soldering an old elbow is of course possible.
I'd not want to use a push-fit onto a previously soldered stub of pipe unless you were able to wipe the solder off very well ( blowtorch followed with damp cloth using a quick twisting wipe towards the end of the pipe); I'd either solder onto the stub, or cut back the old pipe to un-soldered copper.
Sometimes it's easier to cut back some distance and run fresh pipe for the final approach, than trying to bodge the existing approach.