Must the minimum absorbance change for a sample always be at least 0.1 when using a Megazyme kit assay?

No.  The 0.1 change of absorbance is only a recommendation. The lowest acceptable change in absorbance can is dictated by the analyst and equipment (i.e. pipettes and spectrophotometer) and therefore can be can be determined by the user.  With accurate pipetting, absorbance changes as low as 0.02 can be used accurately.
 If a change in absorbance above 0.1 is required but cannot be achieved due to low concentrations of analyte in a sample, this can be overcome by using a larger sample volume in the assay to increase the absorbance change and thereby increase sensitivity of the assay.  When doing this the increased volume of the sample should be subtracted from the distilled water volume that is added to the assay so that the total assay volume is unaltered.  The increase sample volume should also be accounted for when calculating final results. 

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