Green Card from L1B - When can I change company?

[voiddrum]

Hi,

I am on L1B visa which will expire by 2017 and will go for 2 year extension if situations permit.
My company is ready to apply for my green card and have started the PERM process.

At which point in the green card process I can switch job?
Or is it that if I am on L1, changing jobs is not an option at all until I get my I-485? - What are the approximate timelines?

Thanks,
Kiran