When faced with a long tail keyword phrase use proper grammar for your site. Long-tails can be misspellings but that is not how you want to be found. This does not mean you can't play around a little with the keyword order if it makes sense when you read it. You want to attract clients who can tell the difference in your products. You want to capture the attention as quickly as possible.  A trick that might be palatable is to reference the correct grammar and in a non judgmental way against the incorrect grammar. ' Some people call our 'widget'', a "widgette", or "widgetes" however the proper reference is "widget".

The point is not to frighten. The point is to plan very well and have a good reason for doing it. Many times a well performing web site is deleted to replace it with some kind of flash monster.

With good planning:
mapping out all the inbound links,
content,
pages,
internal linking,
redirecting them with 301 header,
making sure the web server can support re-directs easily,
URL structure from old to new,