@Matthew Ford as long as you update to an Atlassian account using the same email as your Trello account, there's no hassle.
Typically what has happened in this scenario is that you've updated to an Atlassian account using a different email address than what your Trello address users and it's created a second account.
You should still be able to login to your old Trello account using your pre-existing email address.
The only troublesome bit is if you've created an Atlassian account with your desired email address, you won't be able to change the email in Trello to match and then update, until you've deleted the offending Atlassian account, waited 14 days for it to actually delete (ie. there's a retrieval period in Atlassian accounts), and then you can run the update process again.
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