Just be prepared that you might need to replace your RD after all. DA7900 specs for a maximum 28T rear cog. It's not uncommon for people to successfully put on a cog one tooth bigger than recommended, but two seems to be pushing it. And if the RD is the one that originally came on your bike, it's probably an earlier series - I don't know when Shimano came out with each series, but DA7800 specs for a maximum 27T. Since your mechanic suggested it, try it, see if it shifts okay and doesn't drop the chain, but be prepared.