Will these become 787-836 and 787-936 on delivery? Likewise the 747-8I for Lufthansa will they become 747-830I on delivery?
Yes, although airlines and more than likely those airlines' onboard literature for example will continue to refer to them as just -8 or -9, but they will travel through the factory and will be certified as -836s or -936s, the only reason that the names can work so easily for Boeing as single digits is with the fact they have no customer numbers to attach, and as BelArgUSA rightfully stated, the code also serves as a way to track the equipment throughout it's life.
Why have Boeing decided to start with series 800, 900 and not the more logical series 100, 200, 300 etc, Similarly Airbus A-350 will come in -800, -900 flavours to start with, with -1000 and -1200 to follow.
Airbus started all this dodgy numbering with the A380, they set the initial version as the -800 (despite A3XX concepts being -100, -200 etc) because, they stated, -800 gave the impression of a mature product straight out of the door, whereas an A380-100 would suggest the opposite, while we're on the subject, the 8 in A380 was selected for two reasons, the digit symbolised two decks and it's a lucky number in Asia, a region that Airbus needed to get onboard the A3XX bandwagon bigtime to get closer to the projected long term success.
Anyway, it worked well for Airbus so the 7E7 followed on, again despite the original studies carrying different names, in that case it was the SR, LR and STR and everything since has continued the naming, -8Is, XWB-900s etc