gearbox issue

[Trev.m]

hi im a new member i have got an audi a4 b7 1.9 tdi 5 speed with gearbox issue wont go into 1st gear when stationary unless you turn engine off the select 1st you can then pull off in 1st and if rolling can select 1st at around 5-7 miles an hr so looking to replace box just wondering if i need to use the same code or can i use any or is there a certain compatible code to use would it be much carry on to fit 6 speed box as there are more 6 speed than 5 speed available any info would be great thanks....... newby....trev.m