If your guitar feels good to play, and the general condition is still good, sure can.
Changing pickguard and electronics will not affect your guitar in anyway, so its fine to do so.
Even if you upgrade the tuning machine heads, its reccomended. All these are within the norm to hop up a guitar. You might even be keeping that guitar for a long time too. So upgrading parts is as natural as buying a new guitar.
Unless you are doing something major like fitting floyd rose or something, then have to consider. as some of these fix will cost as much or much more than the worth of the guitar itself.
But some people keep their guitars for life, so it's a personal preference. Do note that by upgrading parts, it often does not increase the resale value of your guitar.