In addition to kopischke's answer:
Since the matching is done by email address - if the email in the commit matches your email, it will do it automatically.
Ideally, this email replacements and fixing should've done while converting from SVN to Git. If using tools to convert it, there are also options to map each email/user to another email that will be written in the corresponding Git commits.
- I did it for my organization when moving projects from SVN to Git, and it worked great. Though I had to map a lot of address manually since the scripts I created to automate it couldn't catch all the cases, but everything worked and each email in any commit was associated to an user in the organization.
After the project was uploaded to GitHub (or any other Git hosting service) or shared with others in any way - it's much more problematic:
It's possible to edit all the commits retroactively locally, and supply the information you want (i.e the new email), but you'll have to force push the repo and everyone will have to force pull it.
It's plausible if the project has small amount of members/contributors and no forks, and then you can reach each of them and arrange that change.
I really don't think GitHub offer a tool like that, but, maybe if you'll add the email in the commit as another email to your GitHub account - it will recognize it - worth trying if possible.