Under TUPE, the terms and conditions of your previous employment have to be reflected in the T&Cs provided by the new employer. So it wouldn't be uncommon for the new employer to write a letter confirming the key points of your new employment but it should reflect the same key T&Cs (well broadly similar) at the date of the transfer.
So what has happened so far is fine, provided you are happy that things in the handbook and statements in the welcome broadly replicate what you enjoyed before.
I really don't know what this 'trial' was supposed to be about; the alternative to not deciding to take you would have been to have sought to declare you redundant.