Hi @Alyx,
Generally you can increase your allowances mid contract by doing a change of priceplan on the Three App or My3. Often with 100GB plans these were given on a promotional basis though, so you may not be able to change that plan.
With a request to lower the plan, you won't be able to do this, but if there's some financial difficulty or other reason you need to do this, then you can always check with the team if an exception can be made.
Customer services should be able to clarify either way if these changes are possible. Sorry that we can't give a definitive answer here, the community is a public space, so we won't be able to access your account here, and most members are fellow customers offering their opinions and experiences.
Thanks,
Jonathan
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