Is it advisable to change kewords after a while and what are the effects of it?
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Yes, you can select the asset from your Dashboard page, and an edit panel will open up below the image where you can change titles and keywords. I have Re-edited some of my images in this fashion when I come across some that can be described more thoroughly. It may take some time, perhaps hours, for those changes to be indexed and available for search. Presumably, this should help the chance for your assets to be found buy a Buyer.
From time to time, I change my keywords, mainly to correct errors of my early submissions. I also adapt the titles, because searching has changed, and the title is now included in search, so I get much more descriptive. However, changing keywords and titles on a lot of assets is quite time consuming, so best is to start with good data from the beginning.
Your keywords influence the searching, so the effect may be huge.
I also change keywords and titles. Especially if an image I think should be selling, but is not.
Changes taske place immediately, however it may take several hours, even a day, for all servers to be updated.