I think those who are concerned about the amount of money and time sunk into marksmanship are looking at it in the wrong way:
You put in-game money and time in for an advantage; that advantage has now been taken away.
I do not think a financial reimbursement is the way forward (as it is a 15-level difference), but they should, nonetheless, be rewarded for their dedication to the system that you built and wanted them to use. Potential rewards could be a clothing item, vehicle or some recognition for their efforts.
That being said if the developers do not see it as a problem at all; there is nothing to solve. But if there is a drop in longtime players, it would be inaccurate to say you did nothing about it or made the best efforts to solve the problem.
I do not understand how you
@Ayjay can say it is unreasonable for them to be peeved that they have lost their advantage. In my opinion, they should receive something for it (for the reasons above). It is not my job as a member of the community to figure out a compromise to this problem;
it's yours. Whilst you say that you delivered on a community decision, you need to consider the fact that those people that played the game the way you wanted to them to should be rewarded. Who's to say that other changes will be made which makes the substantial time put into an element of the game; wasted.
To those saying 'shut up and play', this has long been the community mentality which drove members away from the player-base. If you want an increase in players, innovation needs to be made (as done with some of your great new updates), with clear considerations for the players which have played your game for a long time. Acts such as this one are great to level the playing field, but compromises are needed to acknowledge the value of the dedicated.
And this strategy of 'comment point scoring' (linking to thick skin, putting salt images and general vexing comments) whilst great for your reaction rating total, yields little fruit in actual solutions.