Long-term management of rental portfolio

[Balane Frece]

Hi all, 

I hope your week is going well (halfway to the weekend)! I'm curious if anyone could share any literature (or podcasts) concerning how to manage a rental portfolio long-term. For more context, we are in our late 20s and have a handful rental properties that are positively cash flowing (about $200/mth/door). Our intention is to hold them for 60+ years. (After 30 yrs, they'll be paid off and then we will use the enhanced positive cash flow ($1000/mth/door) as passive income in retirement). Now, I know that we won't be able to keep the same properties for that long so my question really is, when is it a good time to exchange one property for another? (My concern is that 2 of our houses were already built in the 70s so I'm not sure I see them standing the test of time.) Is anyone doing something similar? 

Best,
Balane