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I only did it with some trepidation as I'd had a slipping seat post on PBP (my fault - dropped it slightly half-way round to help with saddle sores and didn't tighten it properly) and my knees were very sore at the end. Luckily they didn't rebel against this adjustment though or else I would have been stuffed.
The other thing they say to do is to move cleats back, but it sounds like you couldn't do that either (nor could I - mine were as far back as they could go).
A lot of people had achillies issues and quite a few had to abandon as a result.
My right one started to bother me at the end of day 2 and into day 3, so I lowered my saddle by quite a bit to take the heat off it, just before heading up into the Jura. At that point I was starting to think it might be a ride-ender, but the change fixed it, or stopped it getting worse. It's still a bit stiff even after a few days rest. Funny, as I'd never had an issue with it before.