How to change default mappings

I start vim-taskwarrior with the following commands:

```
TW

nnoremap <buffer> s :
nnoremap <buffer> n gk
nnoremap <buffer> r gj
```
Afterwards I can move lines up and down with `n` and `r`.
However, after syncing by pressing `S`, only `n` works as expected; `r` was reset to change report type

How can I change the mappings for vim-taskwarrior permanently?