For a M&P 9c.  

I need to modify 1 magazine to 10 rounds or spend $25 for a neutered magazine.

How can I legally modify an existing magazine to be complaint?  Is it even possible?

Did WA get fuck to the 10 round shit vs. the full capacity? If so I will have to my BIL his Sid with 15 round mag has been nerfed. Sad to live in a state the doesn't know standard capacity mags are.

No, will be traveling and don't want to get I'm trouble with Da Man.

I think it depends on the state law.  I'm sure that some places allow different things than others, so any response you get here might not be correct for the state you are thinking about.

I'd ask in the hometown forum for the state that you are going to travel through/in.  You're more likely to get accurate information that you can actually use.

I would pony up the money for a couple 10 round mags honestly. I used to live in a ban state and most states will say the mag has to be permanently modified to hold only ten, which often means welding riveting or epoxy to make it incapable of disassembly making cleaning and maintaining impossible. There is/was a company called mag block dot com making limiting blocks you can insert to limit capacity but you still need to seal up the mag some how to be compliant with law. Having extra magazines around is never bad and if this is for carry you should be carrying a reload any way. While there is some issue with 10 round factory mags in some guns they would still be likely more reliable than anything cobbled together. There was some complaint for example that glock 10 round mags were not reliable but mine have been fine. I at first after I moved figured I would get rid of the gen round mags, but several types of competition I shoot ( uspsa production idpa and gssf) have a gen round limit anyway so have continued to use them.

One thing you could do is grab a chunk of plastic the thickness of a cartridge and cut it to fit in the bottom of the magazine. Drill a dimple for the retention plate's nub, and then drill a second hole on the other side under the hole on the baseplate. Then you just have to pop a screw up in there and you have a permanently attached magazine block.

Get a pop rivet gun from Harbor Freight. I did and can limit the capacity of my Beretta mags to right at 10 rounds.

I believe that is legal in most states as the rivet is considered permanent and the baseplate can't be removed after drilling and putting the rivet in

Colt did this with their AR mags in the 1990's