Trademark renewal is a relatively simple process. Trademarks are generally valid for ten years, after which they can be renewed for another decade. The number of renewals is not limited.
Usually, it's worth starting the renewal process six months before the expiration date, although some countries, such as the USA, Australia or New Zealand, allow applying up to twelve months before expiration.
In the case of a renewal, the trademark in question is not assessed again.
The renewal can be requested by the client or their legal representative. It doesn't have to be the attorney who did the original filing, so you will be able to change the representative for the renewal process if preferred.
If you miss the deadline, in most jurisdictions, you'll still be able to file the renewal application within six months after the expiration. This is called a 'grace period'. After that, the owner of the trademark automatically loses the right to their trademark, and if they want to protect their brand again, they have to go through the whole registration process again. When applying for renewal during the grace period, special conditions and additional fees may apply.