when in gnuradio top block is started can we change connect blocks?

[sp]

I have a question about gnuradio blocks when I define gnuradio blocks on a top block and start top block can we changed connection between blocks when tp is running with out stoping it.....

for  example i connect uhd to a null block, after some time i changed uhd to another block?????

can any one guide me?

thanks in advance