I have use Veritas NetBackup and we use Veritas BackupEXEC
at this site too.
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1)    Customer has already been using other backup software/method such as
Veritas or Legato or tar or cpio.  Can anyone share their experience with
me ?  I know one of the ways is to install TSM on the same client and start
the parallel run.  Then slowly migrate to TSM.  But the drawback is the
client may not be able to afford two backup agents running concurrently.

Yep, the good thing is after TSMs first backup, the use is pretty light
compared
to other methods with fulls and Incrementals or Differentials.  The trade
off is
dependence on the TSM database. TSM makes good use of its tape drives, where
the direct to tape systems, must have tape drives available during the
backup
period.

NetBackup uses a neat multiplexing method to put data on tape, and TSM also
mixes the data that is put on tape, but this is after it is typically when
writing from its disk pool.  Amanda uses
a disk pool o hold fulls and incrementals or differentials till you send it
off to tape.
I did help with using Legato, and it seems like more of a competitor head to
head
with NetBackup rather than TSM.  BackupEXEC is a little brother of NetBackup
(not
literally, but in a marketing kind of way).  It is another
full/diff/incremental
kind of backup scheme.  I think NetBackup and BackupEXEC both have an option
to use
a disk pool to backup to, but I have not had the opportunity to use this
feature and
I don't know how it could be worked into the backup scenario.  I think then
they would
be a little more like Amanda when used, but that is my speculation.

2)    Customers have some archive tapes that were backup using other backup
software, e.g. financial data that need to be kept for 7 years.  How to
make TSM able to read these tapes and restore them ?  I know we can suggest
customers to restore the tapes first and then use TSM to backup.   But is
there any other ways beside this one ?  Customer may not have sufficient
system resource to restore and backup the data.

My suggestion is to either 1) keep the old system for 7 year is few restores
need to be made (my preferred if it is a LOT of data), or 2) restore and
re-archive
using TSM with appropriate archive length times.  Another option is to do a
short
term lease on equipment to do (2).

Just some thoughts. ... Jack