I have a product template with a color attribute, which creates variants. 
My product template's attribute_line_id for the color attribute contains two value_ids: 500, 501 (corresponding to blue and yellow). I have two variants associated with the product template, one with color=blue and the other with color=yellow.  

Now I want to change the values of the color attribute for my product template and its variants. The new values should be "dark blue" and "soft yellow".  I cannot edit the name of the attribute values, but I have to link the product template and its variants to those new attribute values.
I can update the attribute_value_ids of the variants, but when I do it, the product template still shows blue and yellow as colors. If I try to update the attribute_line_ids of the product template, for the color attribute, then new variants are created. But I do not  want new variants to be created. I need the old variants to be updated and still be linked to the product template.

Can I do this with the RPC API? If so, how?
If not, how could I do it?