An 8-speed freewheel will probably be too wide for your hub and rear stay spacing. Plus the cog spacing between 8 and 7-speed, despite being close, may be enough to throw off your indexed shifting.
What is meant by 'tired'? As in the freewheel if filled with crap and needs a flush and relube? Or perhaps some of the cogs are worn and need individual replacement. Both situations can be fixed without resorting to a new freewheel.