Someone who purchases a perpetual license at such a low cost is likely to balk at being forced into a SaaS model.

Split test two licensing models and compare the conversion rate for a SaaS model vs Perpetual license.
Test different variables, price, subscription duration.

You might consider moving from a license per computer to a per user license as this is the very common type of Saas licensing model.

Once you have a data that backs up a move to a SaaS model, then simply discontinue offering the perpetual license. As your old customers use up their license key resets transition them to the SaaS model.

You will definitely need to include free software upgrades in the SaaS model and limiting the number of key resets would generate canceled subscriptions.