What are the steps required to revert from STI shifters to friction shifters?
Basically you may drop steps 2, 3 and 6.  You'll just remove the shifter cables from the STI levers. It might not please the aesthetes but it will work. If the STI levers have the shifter cables run through external housings you remove these as well. If the housings run under the tape, you leave them alone, you just cut them where they come out from under the tape. (At this point it is a good idea to buy a cable and housing cutter which is a very useful tool that avoids a lot of messy and badly cut brake/shifter cables and housings.)

The downside for keeping the STI levers is that the combined levers are heavier and have the unnecessary side-ways freedom of movement. But that's fine if it doesn't bother you. If you find out that this is unnerving then you may switch to regular brake levers.

The day you want to change the tape, you may acquire a pair of regular brake levers and put them on the bar. 

Edit: No, you can't keep the shifting cables running from the bar just through the friction shifters. You'll need to remove them and rethread them directly through the friction shifters.