As far as I know, no.
If you buy the €12.99 membership then you're on that from then on and can't go back to the €9.99 one.
But, at a glance, I can't find the details on the terms and conditions.
You might want to check with 48 support to be sure.
See the link in my signature below or email them at support@48months.zendesk.com
According to this, the €9.99 membership is no longer available since April 2022 and users of that membership were transferred to the €10.99 membership...
https://48.ie/legal/price-plan-details
From the 5th of April 2022 the €40GB €9.99 membership will no longer be available to purchase.
For any customer currently on the €9.99 membership after the 5th of April 2022, you will be moved to the €10.99 membership, increasing your monthly data allowance from 40GB to 100GB Data and EU Data Roaming from 9GB to 10GB.
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