Donna was still the same person....just not with the wealth of (impossible) experience but with the ability to become something better.
She wasnt made into a "new person" by travelling (bar the last bit)....she developed.
Who she became was always there, below the original donna surface but she was unhappy, unfulfilled, was made to feel like a loser by her mum and friends, dead end job, single at a certain age...........the lot! Her new personality wasnt "magicked" but was an enhancement of what was there.
Sure, she wouldnt be the person she developed into after travelling the stars ..............(how could she be?) .....but how do we know she wouldnt develop into something more than "a gibbering idiot or pig" with a husband who loves her, money, new found respect from her mother (who treated her pretty badly before), maybe children?
If you look at the episode where she did marry Lee McAvoy and had children.......she changed into a mature woman who lived more than a pig with a new trough i reckon.
and that was without the doctor in her life/memory