Yes, and it is more common than starting on 6 April. Mid-year setup means we bring in your transactions from the start of the tax year so the software holds a complete year, not a partial one — otherwise your final declaration will not reconcile. Practically that means importing bank data back to 6 April, entering anything that did not go through the bank, and agreeing opening balances with whoever prepared your last accounts. If you are already mandated and have missed a quarterly update, we deal with that as part of the work rather than leaving you to explain the gap. The only genuinely awkward moment to start is a fortnight before a quarterly deadline, and even then we would usually file first and migrate after.