If you hack the core of k2 and change it, you will most likely not be happy with your upgrade or your data after the upgrade when the time comes , hence create and override (thats what k2 is good at) and change the way your data is displayed.
getk2.org/documentation/tutorials/item/174-templating-with-k2-and-the-concepts-of-sub-templates

now I know what you mean, but i guess this override doesn't help at all.

because in some items i use one of the extra link fields, in the next probably two and so on...so I never know which one not to display.

But i don't understand why it is usefull that K2 inputs "http"?
in early versions at least, this plain http was ignored and the extra field not displayed if there was nothing else, but now if i don't type anything i have in every item 5 http under "addidional information.

but why should it make trouble in upgrading? if i just take out this one feature or overwritte the "http" with ""?
the code is still the same, and if there will be an update, i can redo it...