I was wondering how difficult to unsolder and replace one of those.

How do you keep the heat down and what is your experience,, do they hold up ok while soldering with a small tip?

I also have a SMR number 3000 which has what looks like a tiny burn mark on top of a 'zero'
so it looks like the third digit is a burned zero.
3, 0, burnt zero, 0
Does a SMR with a number 3000 sound reasonable? It is hard to tell if it was a zero or another digit, but 0 is my guess. You can barely see the edges of a zero digit using a big magnifier on the outer perimeter of what looks like a burnt spot on top.

 

I would be pretty difficult for me as I can't ID what part you mean.

Generally SMD's can be removed if you don't mind loosing the part. Flow lots of solder over it and try to get all leads to heat up evenly so it all comes up at once.

Put the new one down one lead at a time, and do have some solder wick handy for those nasty solder shorts.

 

Since the package is toast anyway (or will be by the time its removed) I use a pair of very fine-nosed, flush ground cutters and carefully clip the leads as close to the package as possible. Once the package is removed I use a vacuum de-soldering station or for a few, a wick to remove the loose leads from the pads. For me at least, there's less heating on the small pads that way.

To resolder, I tin the pads and put a little extra solder on opposite corners then tack the chip down at those corners and inspect the pin/pad alignment under a magnifier. Once aligned, I solder the remaining pins by heating on top and gently pushing the pin onto the tinned pad until the solder underneath flows using the smallest tip I can get for the iron.

The goal is to minimize heating of the PCB pads, they're small and will float right off the board with too much heat. I'm good to TQFP-64 with this method. It works fine with smaller parts of course.

As ErnieM says, have good quality solder-wick handy. I wait until the end and fix all of the bridges at once.