Should/Can I raise rents right after purchasing this 4 unit?

[Michael McManus]

I'm looking at a 4 unit property now and the rents are low.  Two of the renters have been there for 20 years and the rents have never been raised!  

Can I only raise rents with the renewal of a lease?  

Can I force the tenants to sign a new one with raised rates?

When raising rents, would you recommend I do it slowly over a few years?  

The current rents and where I want to raise them is as follows:

Unit 1: 785/1075

Unit 2: 675/825

Unit 3: 725/825

Unit 4: 550/825

To get max rents, slightly higher than my goals above, I'll have to do some cosmetic upgrades to 3 of the units when the tenants move out.

The numbers I'm getting when running with the current rents are giving me just over a 5% COC return. After the rent raising goals, it jumps to 13%. With the 5% COC and needing to update the units when the 3 tenants move out, I'll have to pour a bit more money into it each time. This is a concern for me as the down payment on this property stretches us thin with our savings. What would you suggest there?

Sorry for all the questions!  This would be my first property and I'm trying to be as thorough as possible in my numbers and plan of action.