The plus (+) character is technically reserved for use in the query string part of a URI. In the URL path itself the character should be encoded, if used - and that gets ugly fast.
2.2. Reserved Characters
Many URI include components consisting of or delimited by, certain special characters. These characters are called "reserved", since their usage within the URI component is limited to their reserved purpose. If the data for a URI component would conflict with the reserved purpose, then the conflicting data must be escaped before forming the URI.
Msg#: 4121221 posted 8:13 am on Apr 26, 2010 (gmt 0)
took over the seo role for a site using "+" as a separator.
convinced them to change to "-" ... even though that meant changing >90% of URIs the site has improved in longtial rankings and is generating a lot more traffic since then. However, some other changes were made at the same time; but the + to - switch was definitely helpful. I left the + separator live for one sub-cat to use as a control. It wasn't important for the site, so they were happy with not having to change the URIs. Other changes were made though. After normalizing traffic values that section seems to get ~10% less traffic in contrast to what other sections gained... but the section is so small, and get's so little traffic compared to the other sections on the site that it is very hard to get accurate data.
Although a plus symbol is a reserved character in a URL, it seems to be treated as a valid word separator nonetheless and IMO delivers equal relevance to a given keyword. The only SEO downside I can think of is that it suggests the pages are dynamically generated, which increases the chances of obscure pages/complicated URLs not being indexed.
I definitely wouldn't recommend changing a plus symbol to a hyphen for the sake of it on an existent site with established URLs unless there is some benefit other than increased keyword relevance in a URL you're aiming for. You'll likely end up with less total value even if you migrate URLs appropriately.