There are two situations, each with their own recommended solution:
You have never logged in to the new platform yet
In this case you can do the following: change the email address of your EU Login so that it is the same as your email address on the legacy platform. You can then log in on our new platform. You will then get access to the account that was imported from legacy.
Do you no longer have access to the email address that was set on your legacy account? Then contact your NSO and ask to 'change the email address of the imported account'.
Do you use your EU Login account also for Erasmus+ activities? Then follow these steps: 1. change your email address on EU Login, 2. log in to our new platform a first time, 3. change the email address of your EU Login account back to what it was.
You have logged in to the new platform at least once
This probably means that you have two accounts: one that was imported from legacy, one that was created when you logged in with a different email address. To discover what must be done, please have a look at this article.
Changing your email address on EU Login will not allow you to access your profile that was imported from legacy.