resorce assignnment effective date

Dear All,

can you guide me for the below mentioned query..

i have assigned the resources of the last year(2008)project.
since the limit (max units/time) for the last project is set for ex.60h/h.
now dor the present project i hace to change it to 40h/h.
now i have 2 options .
either i can edit the limit in the already available units(whose effective date is jan 2008.
the other option is i can add one more unit and set the limit(with effcetive date as july2010)
so now when i level the resources p6 will take how many resources as available.
ie it will consider only the resorces of 2010 (with 40h/h)or it will take 2008 resources also(60h/h).

i think my question is clear.


thank you.....