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.
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: old page ->302 to an intermediate page, disallowed in robots.txt. Then the intermediate page ->302 to the new page. The new page will have to get its own links and start climbing up in ranks just like any other new page.
If at all possible, you should try to get some related relevant content on both pages. 1script suggested that you add a paragraph to the old page that has some relevance to the content of the new page. I suggest that you also add some content to the new page that includes the old keyword and some other words from the old page. This would make the re-direct look more natural. If you can't do this, then you might think about the double redirect that 1script suggested, but that would be my second choice.