I was looking at was to change my actual WordPress theme to ASTRA. I’m worried about if this would have a serious effect on my SEO.
Also, this change may take me some days to do. How would you guys proceed with changing a theme so clients don’t see a website under construction?
You should always setup a staging site to do any major changes.
Generally, changing the theme won’t affect your SEO. Of course this is assuming you aren’t also changing page builders.
The theme only controls the front-end so it shouldn’t. But things might break visually. Like bluesix suggested, try it in staging.
In general, theme changes don’t change SEO. Search engines are looking at what is inside the “house”, not what color you’ve painted the walls (if that make sense).
As has been mentioned, staging site to test and work on any major change.
Check that your new theme is not slow to load by testing the demo in Google Page Speed Insights.
Make sure your content can easily be transferred from your new theme easily, in the most automated way.