As a general rule, an employee's contract remains in force with a successor employer, if the contracts were assigned in the purchase agreement with the new employer. However, if this was a specific asset purchase, rather than a purchase the entire business, then your contract may not have been included in the sale -- in which case, there would no longer be any noncompete.
Determining whether or not the prior noncompete is still in force may not be worth the new employer paying legal expenses to determine. It's simply less costly to demand that all employees sign a new agreement -- which is probably what's going on in your circumstances.
Obviously, if you refuse to sign, then the employer may simply terminate your employment. You could ask for consideration in exchange for your signature. Reasonable consideration for a noncompete under Pennsylvania case law has been found where the amount of the employer's payment exceeds $100.
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