bought partly occupied duplex, tenants month-to-month, change terms of lease

[Dieter Best]

Hello,

I bought a duplex in california. One unit is occupied, tenants are month-to-month, tenants pay for all utilities, except water. The duplex has only one water meter.
For the tenants that will occupy the empty unit in the not too distant future I can make my own rental agreement, in particular, I plan to let them pay for the water as well, and I will set a reasonable security deposit.

But for the tenants in the currently occupied unit:

1) Would it be possible to write an addendum to the existing rental agreement to require the tenants to pay for their portion of the water bill? - For example, if the other unit is occupied with the same number of tenants, 50%.

2) I also would like to increase the security deposit. Any suggestions on how to go about this properly?

3) Would it be possible to make a new rental agreement with the current month-to-month tenants, so I would have them on a rental agreement with my terms?

Anyone any experience with this sort of situation?

Thanks a lot in advance.