^This and only this!! Anything else could potentially ruin the jacket and render it useless. Cut your loses, sell the jacket and buy one that fits. You have the perfect template for any adjustments you need to make right now. Base your numbers for the next one off this one and then sell it! Roll those funds into your next jacket. This way you hit the ground running, if you soak and ruin it you're starting a full two steps back....
I agree with most of the comments above and fear what it might do to the jacket. What type of lining do you have? An option is to send it back to Aero and have it relined with a different lining. Shearling, quilted wool, and corduroy will all take up a bit more than a standard lining and may solve your problem with an added benefit of wearing it into cooler weather. I did the opposite with my Teamster changing sending it back for a cotton drill relining and it made it fit a bit more roomy. I have corduroy on my FQHH Teamster and cotton drill on my Vincenza one, they fit very differently.
Show us some fit pics!