[Help-tar] Merge incremental backups created by tar -g

[CUI Hao]

(forgive my poor English. I'm not native)

As is known, I can make incremental backups using tar:
   tar -g testbk -cvpf test0.tar test
     (After test/ is modifyed...)
   tar -g testbk -cvpf test1.tar test
     (...)

As time goes by, a lot of archives is made. I want to merge some old
backups to one archive.
I simply tried to restore old ones and re-archive them:
   tar -g testbk -xvpf test0.tar test
   tar -g testbk -xvpf test1.tar test
   tar -g testbk -cvpf test01.tar test

It seems that test01.tar works well to replace test0.tar and test1.tar:
    tar -g testbk -xvpf test01.tar test
    tar -g testbk -xvpf test2.tar test
    ...

As I am not sure about the content of data file (testbk in above
example), I wonder if there is any side effect to do so? Can this lead
to files being restored incorectly?