I don't think I'd want to cut that quarter like that. I'd do whatever I had to, to avoid that.
If I was doing it, I'd want to verify where that line is compared to the front fender first like MJM said. Then I would de-skin the door. I just don't see any easy way to do it so de-skin the door. I would mark out the new line with string or a yardstick. Then tap lightly with something in portions across the line to mark it so you could see it on the backside and use a marker on the backside to show the center of the line. I use a small blocking hammer and a mallet, other things would work like a blunt pointed chisel. Then using a blocking hammer like this:
I would block on the back side of the skin into a sandbag the new "line". Once I did that I would then use a hammer and dolly to create the line in the correct location.
Using the dolly pictured you would hold the pointed end where my finger is in the center of the new line on the underside and hammer the blocked metal into the line.
Easier than it sounds, but I don't know if you want to get into all that or want to buy the tools needed. Thought I would post it anyways, maybe someone else will find it useful. This is an abridged version I left out a few details.
If I can think of an easier way I'll post.
If I was doing it, I'd want to verify where that line is compared to the front fender first like MJM said. Then I would de-skin the door. I just don't see any easy way to do it so de-skin the door. I would mark out the new line with string or a yardstick. Then tap lightly with something in portions across the line to mark it so you could see it on the backside and use a marker on the backside to show the center of the line. I use a small blocking hammer and a mallet, other things would work like a blunt pointed chisel. Then using a blocking hammer like this:

I would block on the back side of the skin into a sandbag the new "line". Once I did that I would then use a hammer and dolly to create the line in the correct location.
Using the dolly pictured you would hold the pointed end where my finger is in the center of the new line on the underside and hammer the blocked metal into the line.
Easier than it sounds, but I don't know if you want to get into all that or want to buy the tools needed. Thought I would post it anyways, maybe someone else will find it useful. This is an abridged version I left out a few details.
If I can think of an easier way I'll post.
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