I’ve just been reading a book called ‘the organised mind’ which has some pretty good up to date research on memory and attention.
Your description of memory decorators are a good analogy. Memory are formed when we’re attending (focusing) on something and are encode with the situational and emotional data we’re experiencing at the time.
Every time we recall a memory they are augmented by new information, our present thoughts can change the nature of the memory I.e. If you recall a good memory when you’re sad it will be recalled as slightly sader next time your recall it, for example.
In addition as our memory is a big association machine every time we encode another memory new associations can be encoded into the old memory. (Think of it as constantly adding new meta data to existing records)
In memory terms your friend becoming a competitive swimmer (although not at the time of the recalled incident) is a function of a the new memory and relevant association recoding the older memory (the additional meta data has been associated and recalled with the older memory).
So when you recall the slide incident you will recall the new ‘competitive swimmer’ association. The two memories are confabulated, not fictions, you are pulling back additional, newer information.
I recommend reading ‘The organised mind’ and ‘Thinking fast and slow’ as that will give you some additional knowledge to support your hypothesis.
I look forward to seeing your thoughts develop.