Can I reuse my consular legalized birth certificate for my Guardian Visa extension in Thailand?

I wouldn't think you'd need to get those re-done every year and that just having the originals from the first extension should be enough BUT I'd still go talk to your immigration office about it well before you go to apply for the extension so you know what they want you to do as far as documentation.

Also remember the first year of a 'guardian visa' <-where you're raising a foreign child here, the 500K baht has to be in a thai bank for 30 days before you apply, but for subsequent yearly extensions it has to be in for 3 months before you apply.