What are linear placed shingles?

There is no valley,flat open seam.the 3 tab side runs from gable towards gable.linear for a lack of a better description.

I assume you will need to remove 3-4' of the existing shingles along your gable ends so you can remove the decking and build in the 2X6 framing for your new gable overhang. If the existing shingles are still flexible and in good condition a good roofer should be able to cleanly remove the old shingles and replace with new shingles that extend to the edge of the extended roof. The roofer basically unnails and removes the old shingles one-by-one instead of using a roofer's shovel and ripping them off as would be done during a complete re-roof job.