Should I resubmit a modified computer science conference paper to a journal?
When in doubt talk to your adviser. 

It is common in the Computer Science that the delta between conference and journal papers is 30%. That is not updated text only, but 30% of new substantial material, such as new experiments etc.

When submitting paper to a conference, there isn't a 30% rule. Your new paper needs to have a stand-alone novel contribution. 

The reason is that CS adopted an iterative publication style workshop -> conference -> journal. So if your extended paper does not get accepted to a journal, you can attempt another journal, but not another conference.