Change names of files in another diretories

Hi I got 2 structures of dirs with files. Dir a and dir b. Now I want change names of files in this dirs. Like u see in path a/1 files got names from 1 to 3, in path b/1 files have name from 4 to 6. How to change name of files in dir b on pattern names of files from dir a? Number of files in dir a and dir b is the same, files are the same, but they have another sufiks.

├── a
│   ├── 1
│   │   ├── file1
│   │   ├── file2
│   │   └── file3
│   ├── 2
│   │   ├── file1
│   │   ├── file2
│   │   └── file3
│   └── 3
│       ├── file1
│       ├── file2
│       └── file3
└── b
    ├── 1
    │   ├── file4
    │   ├── file5
    │   └── file6
    ├── 2
    │   ├── file4
    │   ├── file5
    │   └── file6
    └── 3
        ├── file4
        ├── file5
        └── file6