This issue was discussed at some length on the Community Forum with Frank Hamelly supplying a SQL script to update the assets.
With my client I took a slightly different tack and used the Fixed Assets Modifier tool in the Professional Services Tools Library (PSTL). This tool works on a single asset at a time and, unlike some of the other modifiers, does not allow import of a spreadsheet.
However, PSTL is written in Dexterity and allows the use of Dynamics GP Macros; so after doing an extract of all Asset IDs I was able to use mail merge to generate a macro to update all fixed assets containing a hyphen in the Asset ID.
I have checked Microsoft Dynamics GP 2015 and a hyphen is still disallowed in the Asset ID field.