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A: If your goal is to separate the existing S/D systems to comply with IEC 61511/ISA 84-2005 with all these additions of proposed components, keep in mind your risk tolerance levels and company policy. I expect it is in line with industry practices to meet the highest level of safety integrity levels.
Points to keep in mind:
A: The answer to your question, based on the fact that the transmitters are being used as a part of a safety system is, "maybe." You need to do a probability-of-failure analysis and determine what, if any, SIL rating the transmitters can have, and whether they should be used in a safety system at all.
There's no real reason, other than conformance to standards, that you can't do what you want to do—but you're opening yourself to point failures in your control system and safety system simultaneously. I'm a cautious person, and I prefer the "belt and suspenders" approach. I'd put in new transmitters just for the redundancy that provides. The cost of doing so is miniscule in comparison to the cost of the current project, or God forbid, the cost of the damage an overfilled LPG storage vessel could cause.
A: I understand the narrative description to indicate that Mr. Fattah has two limited-range transmitters, one for low-level safety and one for high-level. With a limited range, there is little value or need for correction for change in density.
The general tone of the discussion seems to be that they intend to improve the safety of the system. Adding new limited range transmitters for high and low levels would further reduce the dangers. It all depends on the "value of the measurements." A safety analysis should indicate if the required SIL is satisfied and possibly indicate the need for additional measurements.
There may also be a need for inventory management. For this, a wide-range transmitter based on weight and thus, value, is usually desired.
I believe that inventory control and physical level interlock are measurements that are best separated. Use all the measurements in managing the system, but do not compromise measurement robustness in order to save a few parts.