Due to image rasterization, you should not get the same grade results when the resolution changes.
The most sensitive grade component is Decodability which measures bar/ space ratios against the standard. Changing the DPI resolution may affect decodability.
Example: At 600 DPI, wide barcode bars may be 10 pixels and narrow bars 5 pixels, resulting in a wide-to-narrow bar ratio of 2.
At 300 DPI, wide barcode bars may be five pixels and narrow bars two or three pixels, resulting in a different wide to narrow bar ratio.
Generally, the Decodability grade will remain in the same range.
Note: there are cases where the Decodability could be better or worse, especially when the narrowest bar is very small. Higher DPI values produce more accurate grade results but require additional processing and may prove to be slower.