Please help me understand unlogged tables

I can imagine a table that is bulk loaded in a warehousing scenario and then sitting statically could be safe, but maybe the question becomes how could the system know it is unchanged if it isn't logged...

Would a correct solution be to bulk load into an unlogged table and then (fairly cheaply?) set the table back to being logged? 

If I my mental model is correct the table would be liable to being lost to a crash during loading but once set back to logged it is durable. Is that correct?

Is it the case that setting a large table to logged status after loading it unlogged is a reasonably cheap operation?

Thank you,

Alex Stoddard