the switch up to Biggles in Looks Back, asking Erich is he's alright four times in the space of basically one conversation

This absolutely KILLS ME. He spent years making the mistake of assuming that Erich was totally fine because Erich said he was fine, and he's not ever making that mistake again. <3333

As far as interesting turnarounds, I also really love the switch from the way he assesses Erich's actions and his own part in it in Hatchet:

[Fritz]“You were the chief cause of his failure.’
‘He would have been the cause of mine had things fallen out differently,’ stated Biggles. ‘If he chose to take sides with people on the other side of the Iron Curtain that was entirely his affair. He knew the sort of people he was working for. I can’t accept responsibility for that.'

... to the later, retrospective version of it he describes in Gun Runners, in which he pins the culpability on himself:

“How did you come to fall out with them — if the question isn’t a personal one?’
‘They took on a professional German agent who had a grudge against us for winning the war. Through my fault he failed in a certain assignment. For that he was sent to spend the rest of his life in the political prison on Sakhalin Island. I thought that was a bit tough, so I took out a plane and picked him up. That’s all there was to it.’