You can top up whenever you like, even with an active plan and allowance to spare. Please remember that if you top up with the minimum amount for your plan or greater in one top up, with allowances left, and your plan is still within its 28 day cycle, your top up will reactivate the plan and your allowances will reset for another 28 day benefit period. Any remaining allowances at that time will not carry over to your new 28 day benefit period.
If you'd like to add credit to your account and not reset your active plan, just top up by an amount lower than your plan activation charge e.g. €5 or €10 for 3 Prepay 15, €5, €10 or €15 for 3 Prepay 20 and €5, €10, €15, €20 or €25 for 3 Prepay 30.