Is it possible to RELOCK an iPhone?

I am going to buy an iphone, there is this one I have my eye on but it is unlocked. I wouldnt mind it being unlocked only I hear it voids the warranty and makes it so you can't download an update or else it will become "bricked". Is this true? if so then is it possible to have it relocked, exactly as it was before it became unlocked?
 
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