Currently I've configured software RAID 1 on ubuntu machine. However I am planning to add SSD drive just for the OS and keep my main 2 HDD for the storage with RAID. The question is, can I put now everything on these disks and later install OS on the new drive and configure these 2 HDD to be in RAID and keep the data I collected already? Another question, Can I resize or change partition RAID disks later?
Installing a new OS onto a newly installed SDD and keeping your RAID partitions is no problem with all modern Linux distributions, e.g. Debian and its derivatives.
When installing just watch out for the following: - make a complete backup of your raid partition onto an external drive and disconnect this afterwards - make sure that you install the root file system on your new SDD and that grub is installed to the sdd.
Could you give more information on the partitioning and directory structure of your raid 1 harddisks? It may very well the better approach to make a second backup, repartition using raid 1 + lvm and restore the data.
re: resize. Raid partitions can not be resized without repartitioning. I would recommend using lvm on top of raid 1, which gives you incredible flexibility and data security.