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Technically, as you never dissolved the corporation, it is still active and technically you could still use the old EIN for both companies. However, I would not advise this
Particularly with an S Corp, because even if you have no income and no expenses, you're still responsible to file an S corp return annually
the penalty for failure to file is $195 per month, per member
All of a sudden starting to use that EIN again may trigger unwanted notices and give you headaches you don't want
So, I would formally dissolve the old corporation and put it to rest, and open up a new business for your new endeavors
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