Anyone have experience (or know anyone with experience) moving from a mostly remote org to a fully / mostly in-person org? We're thinking of moving from mostly virtual to mostly in person. It’s super hard and I want to learn best practices / mistakes to avoid, thanks in advance!

From my experience going fully in person (from remotely) could be risky decision. Definitely should be clearly communicate this decision with good business reasoning.  I know a lot of people who are really satisfied with possibility to work remotely (at least partially). It helps them to better organize their life and work. Don't forget that some people are also loosing a lot of time travelling to/from job and in bad weather conditions (for example snow) it is much more comfortable and less risky to work from home then to travel to work. If you really think to go fully in-person org I would leave possibility and option that few days/months employee could work remotely. 

Nowadays, a fully/mostly in-person org only makes sense when personnel need to collaborate or engage with customers in a real-life activity.   An obvious example is in engineering physical products where the staff needs to create and evaluate solutions in person.  The work may also require equipment that cannot be provisioned at staffers' homes, for physical, cost or security reasons.  In another common example, junior personnel need to be supervised and coached in-person in order to learn and perform their work effectively.   I'd be interested in knowing what other scenarios require organizations to work in person, and cannot be managed or monitored remotely.  In any of these scenarios employees should appreciate the value of working onsite with a team.  

It really depends on where you are starting from:
- is everyone geo-concentrated?
- do you have remote people that can't commute (due to distance)?
- are you comfortable losing people because of this decision?
- are you open to having exceptions? what will be the reasoning?

All in all, people will want to understand the purpose and the problem that you are trying to solve with the RTO... I assure you it won't be an easy rollout.