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Now, I am asked to target completely different keyword for the same page.It would be much easier for everybody to simply restock the "blue widgets" if you ran out of them than to change the keywords ranking signals of a page with 1000s of existing, already indexed (and apparently working) backlinks. If the "blue widgets" and those other widgets you want to cover are somewhat related, I would add a paragraph to the old page with a link to the new. If the new widget is totally unrelated, I have no idea why would you bother with the old page anyway - all the traffic to it would be on irrelevant keywords. If you were successful in tricking people to visit it, your conversions would be so low (or zero), it would not be worth the effort.
I would add a paragraph to the old page with a link to the new
If you still wanted to redirect traffic without the PR (and perhaps other signals like anchor text) to follow, you would have to do two redirects