TaqMan style assays typically use FAM as the default dye selection. It has absorption and emission wavelengths that are very close to those of SYBR Green, so your Ct results should be usable. If this is a critical experiment that will be reported in a publication, I would repeat it just to be safe. If the purpose of the experiment was informational, or for something simple like presence/absence, I would consider the data to be valid.