Curious about the collective wisdom of this esteemed group on two potential scenarios: 1) Ever start out an arrangement and decide that while you have a good connection you'd be more comfortable paying a lesser PPM rate? (read: I f'ed up and wish I had insisted on less). If so, what strategies have worked to renegotiate? This is tricky as you have to hope she feels a similar attachment and could accept different terms. I imagine frequency plays a role in this as well. 2) Ever have a sugar try to up the ante after a few meets? I imagine this one is easier to deal with, unless you really dig her. Just say no and if she bails, she bails. Next!

Starting with number 2 it has happened a few times.  My current atf started at 200, soon after asked me to up it to 250, and more recently up to 300. I did so, but reduced the frequency of visits. Otoh I had one last year who insisted on more money and I just let her go in search of it. She simplemwasn't worth it.

As for the first question, I've tried to avoid the situation. If she's really hot and insists on a higher amount, tell her your first visit will include a special bunus. Call it a signing bonus.  I did that with one girl who simply charged way too much but I agreed since I was too horny to think straight. But our subsequent visit was 2/3 the price at my insistence, and she was ok with it.

Personally I am not one who wants to maintain a long term arrangement, so it is not an issue I've dealt with, but I assume you'd be best off changing the terms of the arrangement so that a direct comparison is hard to make.  

I have successfully renegotiated rates with some SBs.  I’ll claim business is bad.    My current long time go to I took her down to 120 ppm from 250 And she is happy, enjoys our time together, is getting better and better.  Pleasant Surprise on my last visit was I had a bit of a mess all over my little friend and she would normally clean me up with a wash cloth but nope, just licked me clean.  Gotta love a girl that gets into her work.  Then hopped on for another ride.  
Now I’ve had the take me up the negotiation escalator and if they are super hot I’ve done it.   Until....I get pussy that stinks like Pikes Market in Seattle.  The nano second I get rotten pussy ( it only happens on the high dollar girls) I am gone.    

Definitely RE negotiate if you can.  

The core of your question is, as always when talking about sugar, how do you establish the value of your contribution to her life?  In sales, you learn that every time you interact with a customer you are selling them on the value of their relationship with you.  An SB, like a customer, always has the option to buy somewhere else.  Every time she interacts with you she revisits her decision to be your SB. If you want to keep that relationship, you must demonstrate the value of staying with you, rather than leaving.  There are many strategies for this, and some will fit your negotiation style and the current circumstances better than others.  
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To find the best strategy, first, consider how and what value you bring to her:  
1. Why does she need sugar?
2. Why are you a better SD than others? (Trust/respect, quality of sex, flexibility, safety, fun, etc.)  
3. How much time does she have for sugar vs other activity?
4. Does she enjoy you more than your money, or your money more than you?  
5. What will it cost her (in time, effort, safety, funding) to replace you (or just leave the sugar bowl)?  
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Next, what do you value about her?  
1. Why do you need THIS SB? This is not a trick question. You can get laid elsewhere. What's unique or at least rare about her?
2. Why do you really want to change her allowance? Is she too expensive for the value she brings to you, or are the funds just too high to sustain with your budget?  
3. What will it cost you (in time, effort, safety, funding) to replace her?  
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Once you have these answers in your mind you can formulate what you need to change and why.  Now you can modify your continuing negotiations with her with a clear vision of what you want and how to demonstrate the value of that new relationship to her. Then sell it to her based on that value proposition.  
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Ultimately, be prepared to lose her when you want to change the terms. If you don't think losing her is an acceptable risk, then suck it up and adjust your budget in other areas to fund this ongoing arrangement. But understand that ALL SB arrangements have a shelf life. They will ultimately end. So don't make the mistake of continuing to drop cash in excess of the value you receive just because she's already there.  
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Life is good.
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The Cat

This is superb advice. Words to live by.

This is, indeed perfect advice. Best of all, it's free! I am not in sales (indirectly, yes; traditional, no) but this hits home. It is absolutely true, all sugar relationships have a limited shelf life. Right now, she's very much worth it; down the road, maybe not. I was mostly musing with my questions. Thank you.