Substituting HDDs in MD1200 and MD3600f for larger volumes

Hi, We have a storage array consisting of 9 md1200 chassis and 1 md3600f controlling the array. The use identical WD 3TB 6gbps SAS HDDs which we have managed to substitute for the same model or the equivalent from Seagate through these years. Now our supplier told me that these HDDs are no longer produced, and the closest he can find are 4TB 12gbps SAS HDDs. I have two questions about this. 1) Can I use this disks, mixed with the old 3TB ones, in the same volume? I understand that the bandwidth and size would be limited to 3TB 6gbps, but I wonder about the stability of this configuration. 2) Suppose that (in the future) i substitute one by one (after reconstructing the raid) the HDDs on the same volume to, lets say, HDDs of 4Tb and 12 gbps. Is possible to enlarge the raid and volume to the extra size without data loss?. If that was the case I may consider buying larger HDDs and substitute them in the following years. Thanks in advance