How to change the product type of selected Drupal Commerce products (without direct DB updates)?
For sure I'd not use the approach of changing things in the Database directly. Instead I would try to use the Rules module to create a clone (using the new product type) of the selected products. A solution that could possibly work is to (also) get the Views Rules module involved. Some details about this module (from its project page):


  Provides Views directly as Rules actions and loops to seamlessly use view result data.


If you're not familiar with Rules (yet), the previous quote may seem a bit cryptic (it may make you think like "so what, how can this help me?"). Therefor some more details about how to move forward using these modules:


Create a view (using Views) so that you have 1 Views result (row) for each product of old product type and for which you want to create a similar one with the new product type. This view should have fields (columns) for each of the fields from your old products that you want to use to create your new product (with your new product type). Important: use a Views display type of "Rules".
Create a custom rule in which you use the Views Rules module to iterate over each of these Views results in a Rules action, using the Rules technique known as a "Rules Loop". If you're not familiar with Rules Loops, then have a look at the tutorial titled "Lists and loops".
For each iteration step in your Rules Loop, perform a Rules Action to create an entity which corresponds to your product but with the new product type. At that point you'll have all data from each column of your Views results available as so called Rules Parameters (remember: they are the values used in your products with the old product type). So at that point it's a piece of cake to populate the various fields of your product with the new product type.
The only remaining thing is to decide about the Rules Event to use for this rule to be triggered. But that's up to your own imagination (any Rules Event will do). Optionally, you may also want to add whatever extra Rules Condition(s), also up to your own imagination.


Possibly, you also want to delete the product (of the old product type), e.g using a variation of this rule (to delete those products instead of creating new entities). I would definitely do that in a separate rule (not as an extra Rules Action in the same rule), to be able to verify that the new products are created (correctly) before deleting the products with the old product types (better safe then sorry ...).

Note: Refer to my answer to "How to use Rules to send an E-mail after saving a new comment, to users who checked a boolean in their profile?" (the option 2-part of it) for an example (with screenprints) about how to use the Views Rules module.