Ok, so I wrote this whole explanation and the Community broke.
The answer is a lead is assigned to an LP that it's WS can see. It happens ONCE.
You can move a Lead if you want, but you will need to setup workflows in each WS to move leads to another partition.
It works like this;
Lead Partition:
Each WS can see 1 or more LPs. For simplicity, let's look at a 1-1 relationship:
If you have a UK based Form, the lead will be deposited into the UK LP. If the lead comes back in 3 weeks and tries a US based Form, then it will create a NEW lead in the US LP.
To move a lead, the WS needs to be able to access more than 1 partition. You can use the Change Lead Partition action and make this movement automatic, but you have to TELL Marketo to do this.
Read this:
https://community.marketo.com/MarketoArticle?id=kA050000000KyxACAS
Read this:
https://community.marketo.com/MarketoArticle?id=kA050000000KyxC
If you sync with Salesforce, make sure to use the first workspace as your Corporate workspace. New Salesforce leads will go into that workspace, then you can use a campaign to assign them to the appropriate partition