Can one use 过 in conjunction with 知道? I am investigating the use of 知道 in the past tense. As "知道了“ does not clearly mean "knew" but could be more closely translated as "now I know!", a change of state from not knowing to knowing.
How would one translate "I knew"? (As in, I did know before, but now no longer do)

Thanks in Advance!
Do take note that some sentence meaning otherwise under special context.

"知道了"  is a good example. 

Even 知道 carry out the meaning of "know", but "知道了"  in some conversions mean "okay"(noded). 

For example： 

Q：“如果经过超市，顺带买瓶酱油”  If you happens to pass the store, buy me a bottle of soyasauce.

A：“知道了！” Okay （or you can reply "好的!"）


In different dialect such as Cantonese, sometimes it uses as "understood" rather than 清楚了， 了解了, 明白了

我知道了, 你是说如果把醋倒进去它就会溶掉。 Now I understood, you mean if you pour in vinegar, it will melt.  


Because 过 in Mandarin carry the meaning "over/pass"(instead of before) in certain conditions, so you can't use it. As mentioned by @Alex, you can't "un-learn", "de-notify" something.  

In case of "know", to say "I already know that", you can't say, "知道过” ， but "我【已经】知道了”， “我【早】知道了”。 

However, you can say "I learn it before but I forget".

这把戏， 我从前【学过】， 不过我忘了。