Will most probably happen. Puyol will also be taking players away from Barça as a football agent, sooner or later in a different path. Lucho will also not be staying forever and seek new challenges.
Hope no one holds that against them. Don't see why anyone would tbh. Treat as rival for all I care, but won't ever forget what they meant for FC Barcelona nor be butthurt like some for them making the sin to do what people do. Their job.
It's not that probable imho. The chances of Xavi/Puyol coaching a direct and fierce Barca rival in Europe are not that high. And Puyol being an agent is not nearly close to Pep directly competing against us in a CL semifinal. If say Pep would have beaten us last year, for me it would have been an unpleasant memory with Pep going against Barca, and you're not supposed to have unpleasant memories about Barca legends hurting Barcelona. That would affect the way I perceive Guardiola as part of Barca's family. It's normal from my pov.
Same way, if Messi goes to City let's say, and he knocks us out from CL two years in a row, that would affect the way I see Messi as a Barca legend.
If Pep coaches for say 15-20 more years, and he never comes back to Barca, and he ends his career as a manager being more against us, than with us, then hell yeah, of course it would alter the way I see him as a Barca legend. I would see him more as an outsider than an insider, at least as far as his managerial career is concerned.
Let's hope that wherever he goes he fails against Barcelona, so we can have happy memories with Pep outside Barcelona as well.
Can't say I was sad to see him coach Bayern when we beat them 3-0.