Best way to break the seal of GAF HD Shingles

Hello - I need to replace the entire first row of GAF HD (not HDX) shingles on my low slope (yes I know it should have shingles) deck roof due to bad gutter flashing install. Its about a 20' run. The shingles are 6 years old. I started this past weekend (sunny but low 70s) and it didn't go well. I stopped at the first shingle on the second row (to expose the nails) after the shingle started to rip. I was using a small metal putty knife inserting parallel with the shingle. I keep seeing conflicting information from using a heat gun/propane torch or 90 degree days vs cool mornings and ice/water. Any advice on the correct way? Thanks.