Is it correct ot use 以前 in sentence with meaning of future plans 我以前打算去北京? Is it correct ot use 以前 in sentence with meaning of future plans 我以前打算去北京?


  我打算去北京 - I am going to go to Beijing


Will it become:


  我以前打算去北京 - Before I was going to go to Beijing


Or do I need to replace "打算去北京" part to also express past tense?
Is it correct to use 以前 in sentence with meaning of future plans 我以前打算去北京?


The simple answer for your question is "YES, it's correct". You can use 以前 in sentence with meaning of future plans. 

However, your English translation below is not that right. 'Before' is not needed because 'was' already connotes the past tense (which 以前 here stands for). 


  我以前打算去北京 - 'Before' I was going to go to Beijing


So the correct translation should be: 


  我以前打算去北京 - I was going to go to Beijing