I’ve had my handguards mounted with M6 bolts on the barend inserts, but I decided to remove them just to see how hard it was.
Turns out, not hard at all and removing them is probably less of a hassle than drilling and tapping them for M8 threads.
1. Drill a hole roughly the size of the indentation on the top side of the bars.
2. Insert a bolt on the threads and tap the insert loose
3. Pull out the insert using the bolt you just used.
4. Check for burs on the hole you drilled, sand away if needed
5. Done (naturally, removing and remounting the grip is required)