No no no, definitely don't upgrade any 300 max Attack exotics at Xur now. You will be able to level vanilla aka 300 weapons directly to the new 365 Attack level when HoW launches. Upgrading now would also reset all the XP you've invested so far but upgrading during HoW will not reset it.
All the details are still kinda sketchy so if I'm off base on this someone please let me know...
From what I saw and read, the HoW Legendaries all start at 331 attack. No attack upgrade bubbles, only perk bubbles. Etheric light bubble on left side upgrades attack level from 331 to 365. Perk bubbles will probably still require XP to unlock and planet mats/glimmer to level up.
In other words, if you have Etheric Light (which is gated and RNG from weekly activities), you can upgrade any new HoW 331 Legendary attack weapon to 365, but won't have the perks active immediately. If you have an old fully leveled Legendary that is 331 (with all perks unlocked and leveled), Etheric Light will upgrade the attack to 365.
As for current exotics, they will probably remain the same post-TDB base value of 302. An additional Exotic Shard will be needed to upgrade a "maxed" 331 to 365 after HoW drops. This makes sense in the context of Bungie learning from the prior mistake, and now will preserve player investment and progression. If you spent time and XP to level up your exotics, they can be immediately made "relevant" in HoW by simply upgrading the attack from 331 to 365 with an Exotic Shard.
I would presume that if you can use that left side bubble on an unleveled exotic (302 base) prior to it being maxed out that the attack value would scale accordingly. Meaning the left side post-HoW bubble is basically just a 34-attack (365-331) upgrade node. If the attack level is 310, if you can use that node prior to all the other bubbles being unlocked and filled, it would upgrade the attack level to 344 (310+34).
The question is whether the HoW Exotic (namely FoAF) will come with base attack of 331 and whether it will have attack upgrade bubbles. My guess is that it will be like the HoW Legendaries rather than the other pre-HoW exotics.
IMHO--Bungie really knocked it out of the ballpark with the new weapon/armor upgrade paths/progression. But like you, I'm not so sure what happens with pre-TDB exotics with 274 base/300 max. I would presume that one would still need to re-Xurbish it to 331 before it can be maxed to 365,, but Deej wasn't really clear about that before he dismantled his "given" 274 base Godhorn in the video.
