Can I add IPs to a running account without downtime

Yes. Buying more addresses is an addition, not a reconstruction. Whatever is already serving carries on untouched, keeping the addresses it was given; the new ones simply turn up in the panel awaiting assignment. That remains true when what you are adding is a different separation grade, or the pool on the other side of the Atlantic.

Why nothing has to move

The reason is structural rather than a promise about care taken. Binding an account to an address is a one-to-one relationship; lengthening the list of available addresses does not disturb any binding that already exists. There is no reshuffle in which running sites are relocated to make room, because no room needs making.

That is unlike most infrastructure growth, where more capacity means a migration window and a maintenance notice. Here the list simply gets longer and everything already on it stays exactly where it was.

What a follow-on order can contain

• More of what you already have. The everyday case: additional addresses at the same grade, in the same region.
• A wider grade. A small number of addresses at a higher separation level, bought for the few sites where that is warranted and sitting alongside the rest.
• The opposite region. European addresses added to an American holding, or the reverse, under one login and one invoice.

Two things worth deciding beforehand

Size. Each order is assessed on its own count, so dividing one purchase into two smaller ones can cost more than making it once. Where the boundaries fall and why they behave as they do is set out in Billing and Refunds. Worth a glance before splitting anything in half.

Cadence. Topping up two at a time is a great deal more bookkeeping than topping up twenty once, and each order is another thing to reconcile against your own notes. Buying slightly ahead of demand usually works out cheaper in both money and attention.

After the additions land

1. Set the new addresses beside the ones you already hold and look for any shared prefix or run of consecutive values. A sizeable follow-on order is precisely where a shallow inventory would betray itself; the check is described in IP Diversity.
2. Run a reverse lookup on a sample before committing anything to them, so you know what history you are inheriting.
3. Assign on purpose, then update your own note of which site lives where.

Moving an existing site onto one

Also possible, also not a rebuild, but not quite as invisible: relocating a live site to a different address is a DNS change and therefore carries a propagation delay. Lower the record lifetime beforehand rather than afterwards and the switch lands quickly instead of trailing for a day. That workflow, and which grade suits which part of an estate, are covered in IP Diversity and IP Classes Explained.