125 BPM is 125 BPM regardless of your LPB Setting. What changes is how many lines represent 1 Bar in 4/4 musical timing and how long the playhead stays on a line.
1 LPB would mean 4 Lines are 1 Bar with each Line being 1/4 in length
8 LPB would mean 20 Lines are 1 Bar with each Line being 1/32 in length
when I write 20 Lines than it refers to hexadecimal counting, if you have set your lines to decimal counting you need 32 Lines for 1 Bar. 32 x 1/32 = 32/32 = 1 Bar 
Default Pattern length can be changed in Song → Song Options. If you want every Pattern to be 4 Bars long just but 128 here or 64 for 2 Bars etc…
Not to confuse you, but each line is made up on even finer steps called ticks, you can set how many ticks one line has in Song → Song Options, this will change the sound of the Repeat effect for example and also allows you to make notes shorter than your LBP setting with the Cut Command, which Cuts Notes off after a certain amount of ticks, so if you are working in LPB 8 and you want to have a note that is shorter than your 1/32 step length, let’s say you want to have it be 1/64 and you have set your Ticks per Line to 12, than you would have to use C6 as an effect command to cut the note off after 6 ticks to have it be 1/64.
Also: If you recorded your melody with LPB 8 and then change your LBP, your melody will be out of sync and won’t automatically adapt to the new LBP setting (but renoise does offer a shrink/expand option so you can double your or half a selection of notes). Some Breakcore artists work in a high LPB Count like 16 or 32 btw, I think that’s mainly for timestretching effect using the Sxx command and in general i not necessary.