Is it true that if i update my phone software through SEUS, it will upgrade my phone's CID to 53?

If true, is this really that bad? Will i still be able to flash it again (downgrade, for example)?

This applies to (almost) all DB2020 phones, other phones are not affected.
Downgrading is possible, but not for free.
It's not that bad anymore, because all flash/patch operations can be done with freeware as well. But you have less opportunities, meaning less software to choose and the patching procedure is more complex.

My phone is a W760i, DB3150, i belive. Should i update-it via SEUS?

Go ahead without being worried.
You could also flash it yourself if you'd like to.

Note that W760 won't be updated to CID53

Basically that's what I just said

True ...

Thanks so much guys!