Our company is using Altera EP4CE15F17C8N on our printer's controller board and we are having part shortages now.
I am thinking off using higher speed grade part, Altera EP4CE15F17C7N as an alternative.
1) What is the major differences in terms of product spec?
2) If I am using the same external clock, will it cause any timing issue when I mount it directly onto the current design?
As you are moving from slower speed grade '8' to faster speed grade '7', there should not be any timing issues. You should get better setup margin.
For hold timing, you may want to rerun timing analysis by create new timing netlist for new speed grade in the Timing Analyzer tool and check the report.