It is possible to have the battery in your PLB replaced. That usually includes a higher quality battery than the original (6-yr vs original 5-yr) and testing and reconditioning of the unit. I sent my ACR to Solution One Marine several years ago. For about $125 they did a great job and had the unit back to me in about 10 days. I did a quick google search and see several potential options for battery replacements.
I had the battery replaced and then gave the PLB to my daughter. Like you, I now have a satellite messenger. I’ve been happy with that, see no need to carry both.