It can be done, but the cost will be similar to a used WM.
The new ejector will need to be installed and the receiver re-blued. And the barrel will have to be replaced with a 3" version.
There's 4 12 gauge 870s here at the moment, two are 2 3/4" guns and the others are 3".
One 3" gun serves for waterfowl. Another is my main slug shooter and venison acquisition tool. I doubt I've ever put more than 10 3" loads through it and a few thousand short ones. It was my match gun for 3 gun for a couple years.
I use 6-10 thousand shells each year. I think I've shot maybe 50 maggie numbs in the last two years and that's heavy for me. I was testing a Saiga.
Unless you're doing waterfowl or turkey, the 2 3/4" loads will do nicely.
You've got a great shotgun there. Enjoy.....
My advice to you is to leave this one alone. Use it as it is and enjoy it.