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Incandescent light bulbs are officially banned in the U.S.
(www.digitaljournal.com)
This is a most excellent place for technology news and articles.
I think to update the string of lights you’d need to change transformer. Household bulbs have a driver in the bulb that converts the 230V to the ~12V the bulb uses. But for that string of lights, they’d need to get an electrician (or someone who knows what they’re doing)
While you may need to replace the whole strand, and can't just swap in individual bulbs, the strand itself has the resistors needed to let the LEDs function, instead of the individual bulbs.