Data analysis - dissertation

I see no harm citing data from other studies, if relevant, but would be wary re changes to same. After all the data in the other study may not itself be raw ,but interpreted, so I suspect it depends on what sort of changes you envisage as to whether or not it is still valid in say support of your proposition.

If you do change the data format/presentation I would very carefully explain why and how, I would also seek permission from those who conducted the original study- you may find, if you contact them, that they can offer guidance re its scope, limitations and comment on your proposed use, which could be helpful.