No, there won't be any noticeable difference in protein expression levels. You should get same amount of protein with C-terminal tag as you were getting with N-terminal. Generally, C-terminal 6xHis tag are more exposed to solvent and you should be able to the difference while purification. In my experience, C-terminally His tagged proteins purifies better than N-terminal, i.e., less contaminant after NiNTA purification as more number of Histidines bind to the beads. After loading your protein onto the column make sure you wash it with excessive amount of buffer and then increase the imidazole concentration either linearly or in steps.

Changing the nature or the position of the tag may affect the production, solubility or purifiability  of a recombinant protein in an unpredictable way. I would suggest checking, and  possibly reoptimizing production conditions on small batches before going to large scale production

Hi,

It depend on protein, if the protein has a membrane-bound moiety on one site giving the lateral protein function or the site is close to catalytic site or the site is close to an important PTM site on the protein, then you should avoid to have Histag on that end.

Often, C-terminal ends are more safe than the N-terminal.

// Beston