What will likely happen is that blizzard will do one of the following:

If they chose to change the range of existing special affixes then those item values will be re-rolled affecting older items. This likely won't happen. Instead they will likely:

Change the "blueprint" of legendaries to use new better special diablo 3 gold. Any new legendaries that drop will use these new better special affixes which may have a different set of mods or a higher range of them. Old items will not be affected as they will still have the old affixes assigned to them.

Change the "blueprint" of legendaries to have additional special or normal affixes. Any new legendaries that drop will have these additional special/normal affixes. Old item will not be affected as they have already been generated.

As you can see Blizzard doesn't really have control over existing diablo 3 items because which affixes were assigned to them and the range of values on those affixes is based on their seed value. Any change to these may re-roll the item and ruin it. They have to be very careful with item changes for this reason. They can change the "blueprint" of an item, but that only affects new items that drop because once an item is created it's very difficult to modify an entire database of diablo 3 equipment and add affixes to it without changing the previous affixes and possibly forcing the item to re-roll its values (possibly ruining it).

If you want to know more about what will likely happen is that Blizzard will do ,welcome to www.angrygold.com.