Shaykh I read that deliberately changing a single letter from the Qur'an is major kufr. Does it only apply if you're claiming that its correct this way or is this unconditional? What if someone was just wrongly reciting fast to recall to someone a verse? What about omitting 'ayn from the salaam etc.

Ustadh AbdulAziz al-Haqqan

A slip from reciting fast, or saying a letter wrongly, is not kufr and not even a sin. Don't fall into waswas may Allah reward you.