It sounds to me that you found the correct solution, actually. I did a bit more digging after I posted for the following:
For Excel 2003, I found that unlike the TrendLine, we cannot change the Name of the Series either because it is not exposed for manipulation (perhaps, I should have checked before my last post). From Excel VBA Help topic "LegendEntry Object":
Each legend entry has two parts: the text of the entry, which is the name of the series associated with the entry; and an entry marker, which visually links the legend entry with its associated series or trendline in the chart. Formatting properties for the entry marker and its associated series or trendline are contained in the LegendKey object.
You cannot change the text of a legend entry. LegendEntry objects support font formatting, and they can be deleted. No pattern formatting is supported for legend entries. The position and size of entries is fixed.
Thus, what you need to do is to use Alias in the DataSource for the SeriesCollection to supply the correct name/LegendEntry Text which eliminates the need to change the LegendEntry Text. This is what I did in my Access Query that supplies the data to the Chart and hence there is nothing in my code to change the legend text for the Series, only for the TrendLine.
This may be different in Access 2010 / Graph 14 but I'll leave it for you to check your Access / Excel 2010 Help.