the woofer must be 4 Ω and the tweeter 8Ω , thus giving a 'nominal' 6Ω
impedance . So if you are using an 8Ω woofer you should double the value of the lowpass inductor to get the same crossover point.
The 6 Ω value is given by the manifacturer for convention: it tells you and awares you that it's not a 8Ω speaker nor a 4 Ω one. You can consider the 6Ω as the average impedance of the whole speaker, being composed by a 4Ω woofer and a 8Ω tweeter; the value of the lowpass inductor is calculated for the 4 Ω woofer ; the highpass net is calculated for the 8Ω tweeter .
Using an 8Ω woofer with that inductor would shift the crossover frequency higher
Such a vague question does have a vague answer, as picowallspeaker is saying.
Let's be more definite. Presumably your original 4 ohm polycone woofers have rotted foam surrounds. You have some (unspecified) 8 ohm units to replace them.
This needs a bigger bass coil for sure. You will also need to increase the input resistor to the tweeter filter to reduce its level to match.
Something like 1mH, 10uF and 1 ohm on the bass filter. Try around 6 ohms wirewound on the treble filter input to match levels. Shouldn't be too bad.
I would reckon these speakers need to be mounted close to a wall. Not much bafflestep there. The 10uF value might need some tweaking smaller down to 6.8uF. Hard to say, because it depends on the woofer, but I'd start with 10uF.
Find a cheap 4Ω woofer and stick it there !
Possibly before the choice try some
What about the tweeter ? if the crossover is set for an 8Ω tweeter, find a suitable one.
make it be 3 dB louder than the woofer, but limit the choice to medium or low sensitivity tweeters .