etiquette for 'dropping' a letter-writer?

[newbie oldie]

I've decided to no longer ask some people for job market reference letters.  I have asked them on and off in the past, but decided that it makes more sense for me to ask other people. 

It's nothing personal against the past letter-writers.  My research profile has changed, and I've also been tenured and so some of my past supporters fit my 'older' profile. The people who are now willing to write for me make more sense and fit my current research profile more.

What's the ettiquette regarding previous letter-writers?  Should I write them and explain that I'm really grateful for their help in the past, but that for the above reasons I'm now asking other people?  Is there any etiquette or good practices to follow on this one?

Or, if as a letter-writer you've been on the receiving end of this shift, what have you appreciated or not appreciated in the past?

[no]

There is no "etiquette" because this is extremely common. People shouldn't use the same letter writers throughout their career, anyways. I wouldn't mention anything and just do it. If they ask why, explain that you are very grateful for all the letters, but that you wanted to emphasize a different part of your record now. But in all likelihood, they won't ask.