Please see: (+) Sage CRM 2020 R1: Cloning a Person and assigning to a new Company - Sage CRM Hints, Tips and Tricks - Sage CRM - Sage City Community

When a person moves company you should keep the existing person record linked to the company, to maintain integrity and business context for the opportunity, cases, communications etc.

Thanks for the info, this looks like it could be close to what we need.

What would the recommendation be if we didn't want to maintain the original existing person record? Clone the person then migrate their opportunities to someone else at the old company then delete?

You may want to consider a data retention policy.  How long do you keep historic data in your system.
BUT this is my personal view, I think where the relationship with a company is still active then any correspondence with a person previously at that company may well still be relevant.  There may well be opportunities or customer care issues that are still 'live'.  There is a field in the person table called pers_status that would allow you to mark a person as inactive.  So flagging them as no longer with the company may be appropriate. 

That makes sense. I guess the issue is that once that person leaves the company the opportunity would be reassigned to another person at that company or be considered dead, so having the old person record remain really does us no good once that opportunity has a new contact at the prior company.

Perhaps we mark as inactive, but would there be a way to prevent several "Brian Smith" duplicate records from showing up when we search for the name? Perhaps marking them inactive could be used to filter the searches somehow?

Personnaly I would create the person in the new company as a seperate record. The data's clean then imo.

You could potentially use a releationhip to link those person records.

I agree, I have always kept the old one where it is - due to all their opportunity / case / communication history. Otherwise if you look back in 6 months time, who was that phone call with? etc. 

Then either use relationship tab, or a simple Adv. Search Select to link one Person to another

You can, but it is done in a roundabout way and I wouldn't do it. I would always leave the person in the old company and mark them as no longer active and then create a new person record in the new company. Linked records for the old person record (such as emails, notes, opportunities, et al) after a move would no longer be relevant in the new company and would give you a data integrity issue.

If you really want to move a person, you can do it. First you need to change a system parameter.

update custom_sysparams
set parm_value = 'Y'
where parm_name = 'SearchforAllPeople';

Then do a metadata refresh. Then go ahead and create your new person in the new company. Now, with that system parameter change you can merge people outside of the parent company. So, merge the two records. I forget which way you need to do the merges. I can't recall whether you go to your target record and pull into it, or go to your source record and push from it. You'd need to test this. Once the merge is done, you can delete the source record if you didn't as part of the merge.

Three pieces of advice here though:

1. Backup before you do anything

2. Do this on a test system

3. I can't stress this one enough... Don't do it. Just create a new record and leave the old one there and mark it as inactive in the status.